Mathematical and Computer Modelling 51 (2010) 1160–1169
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An accelerated-time microscopic simulation of a dedicated freight double-track railway line Antonio Hernando a , Eugenio Roanes-Lozano b,∗ , Alberto García-Álvarez c a
Depto. de Sistemas Inteligentes Aplicados, E.U. de Informática, Universidad Politécnica de Madrid, Spain
b
Depto. de Álgebra, Facultad de Educación, Universidad Complutense de Madrid, Spain
c
Fundación de los Ferrocarriles Españoles, Spain
article
info
Article history: Received 13 March 2009 Received in revised form 31 December 2009 Accepted 31 December 2009 Keywords: Simulation Railway line capacity Railway traffic planning
abstract The goal of this project (endorsed by the Spanish Railway Foundation) is to develop a detailed and flexible accelerated-time microscopic simulation of train movements within a specialized double-track freight line. The virtual line is provided with a fixed section rail traffic controller (like a classic semaphore-based block-system or Level 1 ERTMS), that is automatically operated by the simulation and obeyed by the trains. It also sets apart at passing loops convoys closely followed by a faster one, so that the slower one can be overtaken (at the first station where this is possible, according to the length of the passing loop and the length of the train). Very low timings have been obtained when testing the software with different sets of data, as detailed in the article. We believe that we have developed a very useful simulation tool, as it can return the result of the simulations of different possible configurations of the railway line in a matter of seconds, consequently aiding experts in decision taking about station lengthening. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The main Spanish railway operator, RENFE, faces two problems when running freight trains on the Spanish railway system (after the liberalization of the railway transportation market in Europe, the infrastructure depends on the railway infrastructure authority, ADIF ). One is that the Iberian gauge (1667 mm), used in Spain and Portugal,1 is wider than the so-called international gauge (1435 mm), that can be found beyond the Spanish North border (Portugal adopted the same gauge as Spain).2 Therefore, freight trains entering Spain from the rest of Europe have to change their axes at the border, which is expensive and time consuming. Although there are exceptions3 the actual policy of the rolling stock and infrastructure companies seems to be to maintain the old Iberian gauge lines for freight traffic and commuter trains and the new standard gauge lines for high-speed passenger services.
∗
Corresponding author. E-mail address:
[email protected] (E. Roanes-Lozano).
1 Except in the new high-speed lines, that were built with a 1435 mm gauge. 2 The international gauge is the standard in North America and the rest of Europe, except in the former Soviet Union and in Finland. Nevertheless, several secondary and industrial lines were constructed in narrow gauge (usually 1000 mm or 1067 mm) all around the world. 3 Like the Madrid–Lisbon high-speed line, where works have recently begun, projected for mixed traffic. 0895-7177/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.mcm.2009.12.032
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The other problem is that passing loops at Spanish stations were traditionally built for 450 m long convoys,4 while freight trains coming from the rest of Europe are normally 750 m long5 [1]. Therefore, trains have to be segregated at the border, what is again time consuming and multiplies running expenses (since tracks need be occupied for a longer time, crews have to be doubled, etc.). With this cost minimizing approach, even to allow 1500 m convoys in some selected lines in the future is under consideration [1]. The work presented here starts with the need expressed by ADIF to have a tailor-made software that could simulate in detail freight traffic in a dedicated double-track real line, so that the exact economical impact of enlarging the passing loops at some stations along the line could be evaluated. We have previously developed other microscopic simulations. We could underline a simulation of passengers’ movements within the airport terminal developed for Málaga Airport (this work was endorsed by the Spanish Airport Authority, AENA, and developed in collaboration with the engineering consultancy AERTEC) [2]. 2. General description The goal of the present work is to aid experts in deciding whether it is advisable to lengthen the passing loops at some of the stations along a double-track freight dedicated line or not, and, when this is advisable, to decide which stations should be reformed. Therefore, the computer tool is designed to allow experimental considering of the different possibilities in order to estimate the real increase in the number of circulations allowed by the different configurations. Consequently, the railway traffic control system implemented within the tool pays special attention to overtaking at stations (the speeds of the different trains and the length of the passing loops are obviously considered). Interference among trains (for instance a slow train running in front of a faster one) is also considered. The simulation can be considered as a quite accurate one, for it runs second by second (of simulated time). There is no limit to the number of trains, stations, passing loops at stations, and such. According to the railway company’s requirements, a tool allowing Excel generated files as input has been developed. Therefore the input is in plain text format. The tool is written for the NET Framework software technology. The new Spanish high-speed railway lines use either the German LZB train control system or the European Rail Traffic Management System (Level 1 or Level 2 ERTMS).6 Our tool is intended for freight lines, where sections are fixed and linked to the infrastructure (i.e., which rail traffic controller is based on a traditional semaphore-based block-system or on a Level 1 ERTMS). 3. Input data Next we will specify the data shaping the input of the simulation program, containing the following information:
• About the line: – length of the line (in hectometers), – maximum speed (in km/h) the present infrastructure allows. • About Semaphores/Beacons. For each semaphore/beacon: – its position from the beginning of the line (in hectometers), – the average speed for each type of train (defined by means of the relation between its power and weight) in the section between the present semaphore/beacon and the next one, considering that the train stopped before entering that section, – the average speed for each type of train (defined by means of the relation between its power and weight) in the section between the present semaphore/beacon and the next one, considering that the train was not stopped before entering that section.7 • About stations. For each station:
4 Probably this was considered as becoming Spain’s mountainous landscape, by railway line designers back in the 19th century, when a great part of the Spanish broad gauge railway network was designed and locomotives were far less powerful. 5 Even these ones are much shorter than US and Canadian freight trains. 6 In rough simplification, ERTMS [3] is a digital radio-based and computer-controlled rail traffic controller that allows to substitute traditional signaling (semaphores) by beacons. In Level 1 ERTMS, the beacons determine the stopping points, send information to trains and receive information from them in a discontinuous way (clearance is given following the same rules as in a traditional block-system). In Level 2 ERTMS, the communication sent from the beacons to the trains is continuous, and braking curves are determined according to the speed and typology of the train (in the LZB system braking curves are determined as well). Finally, ERTMS at its more sophisticated stage, (Level 3), locates trains and establishes on real time train-dependent sections of variable length (this length depends on the braking distance, which in turn depends on the train’s features—speed, weight, braking system), thus allowing to package more trains in the same line. 7 Just observe that we are here considering different average speed values depending on whether trains stop or not before entering the considered section of the line. It is quite obvious that any train which has been previously stopped must progress at a lower speed than another one which proceeds its course without any interruption, in view that the first one will take some time to reach its cruise speed.
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– its position from the beginning of the line (in hectometers), – the number of passing loops it contains, – the length (in meters) of its passing loops. • About trains. For each train: – its departure time (indicating the hour and minutes), – its identifier (each train’s number), – its length (in meters), – its weight (in tons), – its maximum speed (that is the minimum of the maximum speed allowed for that type of locomotive and the maximum speed of all the pulled wagons), – its locomotive’s power. The train’s weight and the power of its locomotive determine the maximum speed it can reach in each section. 4. Simulation Both the clearance given by each semaphore/beacon and the position of each train are calculated periodically over a given constant period of time. The simulation program controls three lists of trains: a list of trains waiting for departure, a list of trains on the line, and a list of trains which have already arrived to their destination. When the departure time for any given train arrives, the program moves this train from the first list to the second one. Once a train arrives to its destination, the program moves the train from the second list to the third one. In relation to the trains on the line (trains belonging to the second list), for each step of the simulation the program needs to perform calculations in relation to the state of the following items:
• Semaphores. The state of the semaphores/beacons8 must be updated taking into account the position of the trains. In Sections 4.1 and 4.2 we will describe how their state is updated for this reason.
• Stations. Each station contains passing loops where trains can wait for another train to pass it. At the end of each passing loop there is a semaphore indicating whether the train at this passing loop must wait or not. The color of these semaphores must be updated taking into account the position and speed of the different trains involved. In Section 4.3, we will describe how the state of the semaphores is updated for this reason. • Trains. The position and the speed of each train must be updated taking into account its position and its present speed, and the color of the semaphores on the line. In Section 4.4, we will describe how these attributes are updated. 4.1. Sections The semaphores and stations divide the whole of the line in different sections. A section of the line is thus defined as the region of the line either between two semaphores or between a semaphore and the beginning or end of a station. For the purpose of our simulation, it is advisory to consider trains as occupying not single points in the line, but actual portions of it, with a given length. We will consider the sections to be long enough to allow a train occupy at the most three different sections. The most frequent occasion is when the train is placed in an only section. However, when a train enters a new section, the train temporarily occupies space on two contiguous sections at a time. The train may even occupy three different sections at a time, for instance, when it is going through a station with short passing loops, and the total length of the train is longer than them. 4.2. Stopping points at the entrance of sections Dividing the line in different sections under semaphore control is crucial in order to avoid the risk of collision. As said above, a classic semaphore-based block-system or Level 1 ERTMS is implemented within the system. In the simulation, safety is based on space, not on time, as is usual in a block-system: sections are fixed and those sections protected by red semaphores progress with the movement of the train. As the simulation focuses on improving the capacity of the line, trains do not try to follow an established timetable; only their departure time and their average speed in each section are given as data. Nevertheless these ‘‘target’’ average speeds are not always reached, as trains ahead could be slower and could slower train(s) behind, until they can stop and wait at a passing loop to be overtaken. Semaphores are placed at the entrance of each section in the line and at the end of each passing loop at the stations. In this section, we will focus on controlling the color of semaphores placed at the entrance of a section with no station in it. In the next section we will focus on the color of the rest of semaphores. The color of a semaphore at the entrance of each section (with no station in it) is green by default. It will be red when a train is placed either in this section or in the next one. Thus, in the simulation, the color of semaphores at the entrance of sections with no station in them will be updated in the following way: 8 For the sake of brevity, we shall hereinafter use the word semaphore for referring to a stopping point of any kind (i.e., either a classic block-system semaphore or mechanical signal or a Level 1 ERTMS beacon). Therefore, when talking about green or red semaphores, we will refer to either real semaphores or mechanical signals or to beacons which are allowed to be surpassed or not (respectively).
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• when a train leaves a section, the color of the semaphore at the entrance of the previous section is updated to green, • when a train enters a section, the color of the semaphore at the entrance of this section is updated to red. 4.3. Stations Stations are sections on the line, where a train can wait for another one to pass it. There is a semaphore at the entrance of each station. Besides, other semaphores are placed at the end of each passing loop at each station. The color of a semaphore at the entrance of the station is green by default. It is only updated to red when all the passing loops at the station are occupied or when a train has not finished to enter the station.9 Thus, in the simulation, the color of semaphores at the entrance of stations will be updated in the following way:
• when a train leaves a passing loop at a station, the color of the semaphore at the entrance of the station is updated to green (unless the situation specified in the following item holds),
• when a train is entering a passing loop at a station, the color of the semaphore at the entrance of the station is updated to red,
• when a train has completely entered a station and there are still free passing loops, the color of the semaphore at the entrance of the station is updated to green. Now, regarding semaphores at the end of each passing loop, their color will be red by default. Only one of these semaphores can be green at a time. Whenever there is a train in the section immediately next to the station, the color of all of these semaphores is red. Otherwise, the color of one of these semaphores may be updated to green, in case that:
• there is no train in the section immediately next to the station and the following one, and
• there is a train placed in a passing loop fulfilling one of the following conditions: – either the train is faster than the rest of trains waiting at the station and the trains approaching the station, or, – the train is longer than the station. 4.4. Trains Each train is characterized by:
• Present speed. It is the speed at which the train runs presently. This speed is updated in the following way: – when a train reaches a red semaphore, it stops (its speed is updated to 0), – when a train is stopped by a red semaphore and the semaphore color is changed to green, then the train starts and its speed is updated to the minimum of the following values: (a) the maximum speed of the line, (b) the maximum speed of the train, and (c) the average speed this kind of train reaches in this section (according to the relation between power and weight) when it has stopped immediately before entering this section. – when a train reaches a green semaphore, and enters a new section, its speed is updated to the minimum of these following values: (a) the maximum speed of the line, (b) the maximum speed of the train, and (c) the average speed this kind of train reaches in this section (according to the relation between power and weight) when it has not stopped immediately before entering this section.10 • Position. It indicates the position (in hectometers from the beginning of the line) of the train’s locomotive. For each step of our simulation, the position of each train is calculated on behalf of its present speed and the time elapsed since the last updating (that is, 1 s in the accelerated-time simulation). • Sections and passing loops. It indicates the sections in which a train is placed (it also indicates the passing loop, in case the present section is a station). This information is updated each time a train enters or leaves a section. When a train enters a station, it occupies the first free passing loop of the station (since the switch always points to the first free passing loop). 5. Describing the interface of the simulation program The simulation program here implemented contains three kinds of windows. In this section, we will give a detailed account of each one of them. 5.1. Main window When the program is run, the window shown in Fig. 1 appears. In the above menu we find the following options: 9 This simulation considers that the switches of the turnouts at stations always point the trains to a free passing loop, if any. The reason is that it is oriented to determine the capacity of the infrastructure, not to find routes within stations. 10 Do note that we always consider different average speeds before entering the present section of the line.
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Fig. 1. Main window of the simulation program.
• Open an input file (see Section 3) detailing the input data about the simulation (that is to say, including information about the line, the trains, the semaphores and stations on it).
• Open a new text viewer (see Section 5.2): by means of this, the user can particularly read any textual detail about the simulation.
• Open a new graph viewer (see Section 5.3): by means of this, the user can focus on any graphic detail of the simulation. Occupying the central part of the main window, there is a black square keeping record of the position of each train at every minute (although the simulation is actually being run up to the second). In the lower part of this main window, the user finds three possible options to simulate:
• Simulate a step. Here the position of the trains for the next minute of the simulation is calculated.11 • Simulate all steps. Here a complete simulation of all the trains until the user asks the system to stop is given. • Run quickly all steps. The program instantly simulates the movements of all trains corresponding to the next 48 h. From this main window we can get access to any desired number of graph and text viewers, as may be seen in Fig. 2. 5.2. Text viewers Viewers are windows used to focus on details of the simulation. As we have seen above, from the main window the user may open as many viewers as wished. The Text Viewer contains a white text box for writing instructions and a black text box where the output of the instructions is shown. The output is updated in each step of the simulation. The possible instructions of the text viewer are:
• TRAINS_AT_ORIGIN. For each step of the simulation, the complete list of trains which are waiting for departure is shown.
• TRAINS_AT_DESTINATION. For each step of the simulation, the complete list of trains which have already arrived to • • • •
their destination is shown, indicating the hour at which they were intended to leave, the hour at which they actually left, and the hour at which they arrived at their destination. TRAINS_ON_LINE. For each step of the simulation, the position and the speed of each train is shown. Besides, also the sections occupied by each train are here shown. TRAIN id . For each step of the simulation, the position and speed of the train with its corresponding identifier id is here shown. SECTIONS. For each step of the simulation, details of all sections in the line are here shown. Each section is detailed, along with the color of the semaphores in it, as well as the trains (if any) occupying it. SECTION pos . For each step of the simulation, the section containing the position pos, is specified through: the position of the beginning and the end of the section, the color of the semaphores at the entrance of the section, and the trains (if any) occupying the section.
11 Observe that the simulation internally runs in 1 s steps, whereas the results are screened to the minute.
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Fig. 2. Simultaneous display of different windows at a given moment.
5.3. Graph viewers These graph viewers hold a visual representation of the simulation, with a legible and dynamic account of every feature on the line: departure and destination, stations (with their looping passes, if any), semaphores, and trains. All these elements evolve in a dynamic way, offering an intuitive visual simulation of every piece of information also contained in the Text Viewer. The user is allowed to zoom on any part of this visual representation, so as to verify details such as the portion of a train occupying more than one section at a time, or any other. Features such as each train’s identifier, the present speed, and the running times are also specified for each moment of the simulation. This visual simulation also offers the possibility of focusing either statically on one part of the line, or dynamically on a train’s journey. The possible instructions of the graph viewer are:
• LINE. This command displays a graphic representation of the entire line as a whole. • SECTION p1 p2 . This command helps the user focus on the portion of the line between points p1 and p2, which is displayed between both extremes of the graph viewer.
• POINT p . This command allows the user focus on a precise point p in the line, which is displayed in the middle of the corresponding graph viewer.
• TRAIN id . By help of this command, the user can focus on a desired train, in such way that this particular train will be displayed just at the center of the graph viewer, either if it is stopped, or as it pursues its course along the line (in this latter case the line will be passing by). In Figs. 3–5 it may be seen the simulation of train L2 passing train L1. Note that train L1 waits at a station for train L2 to pass it because the latter is faster. 6. Taking decisions about lengthening the passing loops Our tool has been designed to be useful for evaluating the decisions on whether it is or not necessary to lengthen the passing loops of certain stations along a railway line, according to the current or the planned traffic.12 In this section, we discuss how our tool may help to take these decisions.
12 This need is obviously related to the train traffic: for instance, in the extreme case that there is only one single train during the night hours, there is no need to lengthen any station even if it is very long.
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Fig. 3. Train L1 waits at the station for train L2 to pass it.
Fig. 4. Train L2 reaches the station.
Fig. 5. Train L2 has passed the train L1.
With a simulation, we can detect those stations subject to raise this question. That is to say, we are interested in selecting those stations such that, if their passing loops were lengthened, the output of the simulation would be different (i.e., timings would be improved). Such stations would be those where the three following conditions simultaneously occurred at some instant during the simulation:
• a certain train, T 0 reaches a station, S whose passing loops are shorter than the length of the train, • there is a free passing loop at the station S, • there is another train, T 00 , faster that T 0 , that will have to stop at the semaphore at the entrance of the section previous to S, as it cannot overtake train T 0 at this station and will have to wait until T 0 leaves the station S (just to keep on following T 0 ). Only when all of these happen at a station S, we may raise the question about if it is interesting to lengthen the passing loop of the station S (otherwise, the output of the simulation would not depend on lengthening the passing loops of S). If the passing loops were extended (to the length of T 0 ), the train T 0 would wait to be overtaken by the train T 00 at the station S and, consequently, the simulation output would change. Once such a station S is detected, the tool may be performed just considering that the station S were extended (to the length of train T 0 ) and then compare it to the simulation of the line when S has not been lengthened. In order to make this comparison we may study the following measure: N P
r =
ti∗
i=1 N P
ti
i =1
where
• ti∗ is the time the train Ti has taken to reach the end of the line if the passing loops of S were lengthened, • ti is the time the train Ti has taken to reach the end of the line in case the passing loops of S were not lengthened. That is to say, r informs about how faster the trains are when the station S has been extended. A value r > 1 means that it could be considered a good decision to lengthen the passing loops of S because this would make the trains faster. The greater r is, the more interesting it is to take the decision of lengthening the station S.
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Obviously, other variables, like the costs of lengthening the passing loops at the station, should be also kept in view before taking a final decision about lengthening the passing loops at a station. 7. Accuracy of the tool Since the purpose of our tool is to check the possibility of lengthening the passing loops at a station, the most important question in relation to accuracy is to simulate correctly in which section each train lies at any given moment. That is to say, we are not concerned with determining the exact position of the train within each section, but with the fact that the train in fact lies in some point of this particular section. Consequently, we are only interested in predicting correctly when a train reaches the beginning of each section. In our simulation, this time can be easily calculated by means of the average speed in which each kind of train covers each section (given as input data). When simulating the activity at a real railway line, the average speeds in a section are calculated empirically from the timetables that trains have to follow during operation in the line. In this way, there is no need to input the tool with data about slopes or curves along each section, since all of them were indirectly taken into account when computing the average speed of the section. We have considered that a train develops a different average speed on the same section depending on whether it was stopped before entering it (because the semaphore at the beginning was red or not) (see Section 3). Consequently, we have considered two kinds of average speed for each kind of train in each section:
• average speed when the train enters a line section without having stopped before entering it, • average speed when the train enters the section after being stopped at the end of the preceding one. When a train must stop before entering a new section of the line (because of a red semaphore), its average speed must be affected to some extent by the process of braking. In our simulation, any mention of braking times has been entirely omitted, with no real difference at all in the resulting global times.13 Let us consider a real train which stops at the end of a section (the semaphore is red) at the instant t1 and it has to wait until the semaphore turns to green at a later instant t2 > t1 . In the reality, this train has spent some time braking. However, since we are not considering braking times in our simulation, the simulated train reaches the red semaphore at an instant t 0 < t1 (that is to say, some time earlier than the real train) and must wait for the semaphore to be green until t2 . As may be seen, both the simulated and the real train must wait at the end of the same section until the same instant t2 , before being allowed to proceed. Although there is certainly a difference between both cases regarding the additional time the simulated train would need to wait before entering the next section, in the whole this difference is unimportant, since both trains (simulated and real) will enter the next section at the same time, and (as we already made clear) our simulation is only concerned with the section each train is at any moment, and not with the very point of the section it is on. Regarding comparison of our simulation to reality, there is still one more detail to be considered. In view that we have used real times (empirically checked on existing lines) in order to implement our simulation, it is not surprising that an exact coincidence between real and simulated times will ensue, considering the real lines the way they actually are; so as to get further empirical verification of our simulation times, we would need to compare those with real times referred to hypothetical lines with modified passing loops, but of course no empirical proof can be made to this extent, unless any modification occurred in reality. 8. Efficiency of the tool As it was seen in Section 4, each step of the simulation involves calculation concerning the updates of the state of trains, the color of semaphores at the beginning of each section, and the state of the stations. As a whole, the more sections, stations and, above all, trains running along the line, the more time the program will need to perform the simulation. Among these variables, the number of trains on the line has by far the greatest importance so as to determine the time it will take to perform the simulation, since the higher number of trains on the line, the greater will be the number of updates on stations and sections needed (obviously enough, a situation with no trains on the line would require no calculation at all in order to perform the simulation). In order to study the efficiency of our tool, we have checked it with different kinds of lines in a Intel Core 2 with 1.67 GHz. As may be seen in Tables 1 and 2, times required to perform every calculation depend mainly on the average number of trains on the line. Anyway, these calculations do never take more than a few seconds, and consequently, this tool may be performed several times in order to decide on the convenience of lengthening the passing loops in some stations. 9. Related works There are already many comprehensive railway traffic simulators, like OpenTrack, RAILSIM [4], RAILS2000 [5], Rail Traffic Controller (RTC), among others. For instance, the possibilities of the relevant simulator OpenTrack can be reviewed in [6,7]. 13 In the simulation, we have not considered the unusual case that a train was braking before a red semaphore, and suddenly this semaphore turned to green, allowing the train to accelerate, since it does no longer need to stop. This situation is very rare, and anyway, the time difference between the real train and the simulated train in this hypothetical situation would be minimal.
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Table 1 Time required to perform the simulation in different lines. Average number of trains on the line
4
6
8
10
13
Length (km) Number of sections Number of stations
100 10 2
200 22 3
500 48 8
600 59 10
800 70 15
Time (s)
0.410
0.510
0.889
1.129
1.544
Table 2 Time required to perform the simulation in different lines on which there are a same average number of trains on each line. Average number of trains on the line
4
4
4
4
4
Length (km) Number of sections Number of stations
100 10 2
200 22 3
500 48 8
600 59 10
800 70 15
Time (s)
0.410
0.423
0.450
0.482
0.498
Independently from the aforementioned major projects, there are as well other implementations developed both by industry [8] and academic [9] environments. In some of these cases, the novelty lies within the tool used, like in [10] (this is also the case with the contribution of [11] to the field of railway interlockings, where a standard computer algebra tool as Gröbner bases is used for helping take decisions in a topology-independent interlocking system). But, probably, the main reason which encouraged us to develop a new implementation in this field is related to the need to focus on a quite specific goal. This is the case, for instance, of the simulations:
• [12,13], specialized in subway simulations, where the boarding time at stations is not constant and depends on the passengers’ flow,
• [14], focused on traffic control checking, • ERTMS Traffic Simulator [15,16], specialized in simulations where the traditional semaphore-based block-system (with fixed sections) is substituted by ERTMS (see Section 2). The latter is also the case of the computer tool detailed in this paper, focused on the possibility to increase the length of the passing loops of some stations. In this model, consideration of the speed of the different trains, and the length of both the passing loops and trains, provides the key for decision taking on the part of the railway traffic controller (implemented within the tool) regarding overtaking. 10. Future work A future improved version of this software could consider topics regarding the accuracy of the present simulation, like providing as input (and considering during the simulation, if applies) the maximum speed allowed to cross the turnouts at each station when their switches are in the diverted track position. Another possible improvement could be to consider the influence of the length of the trains in the value of ‘‘average speed when the train enters a line section after being stopped at the end of the preceding one’’ (see Section 7), in view that the whole train has to leave a section under a speed restriction before beginning to accelerate in order to reach the speed allowed in the next section. Regarding the core of the simulation, adding Level 2 and Level 3 ERTMS simulators could be also considered for further refinement and development of this work. Another improvement could be to include estimations of the energy consumption for the different strategies (a field of expertise of the third author [17,18]). 11. Conclusions A brand new accelerated-time microscopic simulation of a dedicated freight double-track railway line is presented in this paper. It includes the simulation of a traditional block-system to include interference among trains. Moreover, due to its specific goal (to experiment in order to decide whether lengthening the passing loops at some stations is desirable or not), the level of detail regarding overtaking is very high. In this way, our tool provides an extremely accurate simulation on behalf of its goal. Acknowledgements This work was partially supported by the research projects TIN2009-07901 (Government of Spain) and UCM2008-910563 (UCM-BSCH Gr. 58/08, Research Group ACEIA, Spain). We would also like to thank the anonymous referees for their most valuable comments and suggestions.
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