Simulation of different railway signalling systems

Transcription

1 Simulation of different railway signalling systems M. Grimm', B.Sandbla<f *Dept, of Computer Science, Dalarna University, Sweden *Dept. ofhuman-computer Interaction, Uppsala University, Sweden Abstract During multiple vehicle motion, the motion of each train depends on the position of the train ahead. A safety signalling system can be defined in terms of the behaviour of two adjacent trains. Although the rules of safety signalling are well established, there are a number of different approaches for its implementation. This paper presents the results of simulated train operation, controlled by different signalling systems. The difference between the signalling systems is based on how the safe distance separating the trains from each other is defined and calculated. The results of our study are obtained by application of the software package "Order Offer Headway Availability Analysis, version 1.36" (OOHAA), developed for Adtranz Sweden and aimed at simulations of mass transit systems. This paper describes an attempt to adapt the simulation tool to study the capacity of a 139 km long section of a single railway main line in China. The benefit of OOHAA is to investigate and show how closely the trains can be run without restrictive messages from the signalling system. The differences between the two traffic environments of a mass transit system versus a railway main line are pointed out. The positions and situations at which the type of signalling system influences the line capacity are identified. The benefits of continuous signalling systems in comparison to traditional fixed block signalling systems are quantified. The basic requirements of a simulation tool suitable for studying realistic scenarios and capacity-related problems on a railway main line are discussed.

2 1 Introduction The aim of all railway signalling systems is to prevent collisions and derailments of trains. Their function is to maintain a safe distance between trains. Signalling involves a combination of locating trains, making decisions regarding train movement and communicating these decisions to the trains. Railway signalling systems are then broken down into fixed-block and moving-block signalling systems. This paper presents the effect of signalling systems on line capacity. 2 Basic terms and definitions The overall performance of a railway network is usually measured in terms of two key parameters: train speed and line capacity. The train speed is important because it directly affects journey times. The line capacity is important because it is a measure of the maximal volume of traffic carried by a line. For scheduling purposes, it is of great importance to determine the time gaps between trains at the start position, called here the initial time difference and expressed in seconds (s). The capacity of a railway line is defined in this paper as the reciprocal of the minimal initial time difference, for which the following train does not experience the restrictive message from the signalling system. The capacity is expressed in trains per hour (tph). When two adjacent trains travel along the line, the following one needs an appropriate distance to perform braking without collision with the leading train. The area behind the leading train to the point, where a restrictive message is presented to the following train, is called the signal shadow (SS). The distance between the front of the following train and the edge of the signal shadow behind the leading train is called the Distance Headway Margin (HM). The time that is required by the following train to cover this distance is called the Time Headway Margin (THM). Q i Q HM SS Figure 1: Distance headway margin (HM) and signal shadow (SS). Contemporary railway signalling systems are based on dividing the track into track sections, known as blocks, and ensuring that trains are separated by a suitable and safe number of blocks. Such systems are called fixed-block systems. In a fixed-block signalling system the rails are divided into blocks by track circuits. The signalling system knows the position of a train only by the relatively coarse measure of block occupancy. The indication given by the fixed block railway signalling system to a train driver is called here a signal aspect. Moving

3 Computers in Railways 111 block signalling systems are also called transmission based or communication based systems. They liken a fixed block system with very small blocks and a large number of signal aspects. However a moving block signalling system has neither blocks nor aspects. The system is based on a continuous or frequent calculation of the safe distance ahead of each train and then relaying the appropriate speed, braking or acceleration rate to each train. This requires a continuous or frequent two-way communication with each train. A precise knowledge of a train's position, speed and length, and fixed details of the line - curves, grades, interlockings and stations, is necessary for calculations. Based on this information, a computer can calculate the next stopping point of each train - often referred to as the target point - and command the train to brake, accelerate or coast accordingly. The target point will be based on the normal braking distance (BD) for that train plus a safety distance, called here a safety margin (SM). 3 Mass transit system and main line environments The theoretical principle of the moving block system is old. However, the great majority of its current implementations occur only within urban rail transit systems, e.g. in Vancouver, Toronto, San Francisco, Detroit, Orlanao, London and Lyon. Urban rail transit system trains are usually homogeneous, with the same length, speed and braking performance. In normal operation within urban rail transit systems, trains follow each other at regular intervals travelling at the same speed over the same section of track; no train overtaking or meeting occurs; all trains usually stop at all stations and the dwell time is constant. There are no interactions with other transport systems. Signalling of a main line is a much more complicated and complex task. The processing power available in modern computers has been used to perform various simulation tasks related to urban rail transit systems (e.g. Gill & Goodman [2], Gill & Sadler [3], Hill & Bond [4], Hill & Yates [5], Pascoe, Rossi, Savio & Scuitto [7]). Most common are steady-state performance studies considering minimum achievable headway and benchmark tests investigating the recovery capabilities of the system when disturbances occur. Few results related to the performance of the moving block system on a main line are reported, one of them is Holgate & Lawrence, [6]. 4 Simulation method The simulations described in this paper are performed for 139 km long section of a single track railway in China, based on the train and track data delivered by the railway research group at Northern Jiaotong University in Beijing. Typical Chinese passenger and freight trains have been studied for operation with none, one and two stops at intermediate stations. Dwell time for passenger trains was 2 minutes and 4 minutes for freight trains. The aim of the simulation is to analyse the capacity of a railway line controlled by different signalling systems. The

4 Computers in Railways III simulation of the train operation depends on its performance, track topology and the speed profile of the line. During simulation set up, the time difference between two trains starting from the same station is defined. During the simulation, the Distance Headway Margin and the Time Headway Margin are calculated in small time increments for trains directly following each other. After running the simulation, the respective locations where the minimal headways occur are presented, indicating the critical capacity points along the track. A positive value of the calculated minimal headway indicates that the following train never enters the signal shadow of the leading train. If necessary, the initial time difference between trains at the start point may be decreased in order to increase the line capacity. A negative value of the calculated minimal headway indicates that during the operation the following train entered the signal shadow of the leading train. This is possible in simulation, because trains are not controlled by the signals in OOHAA In reality, such an interaction is impossible. The following train would be delivered a restrictive message by the signalling system and forced to respond to it. In order to avoid such interaction, the initial time difference between trains starting from the same point must be increased, or the infrastructure improved (e.g. prolongation of stations, changes in signaling layout). The objective of simulations performed for a specific signalling system is to determine the minimal value of the initial start time difference between trains for which the minimal value of the Distance Headway Margin is positive and as small as possible. In other words, the benefit of OOHAA is to investigate and show how closely to each other the trains, controlled by a specific signalling system, can be run along the line in order to: avoid a restrictive message from the signalling system addressed to the following train, achieve as high capacity as possible and move as fast as possible. Furthermore, OOHAA allows the possibility to simulate the train performances controlled by the following five different signalling systems: - Intermittent, - Quasi-Moving Block, - Moving Block, - Floating Block, - Extreme Floating Block. The signalling systems' performance can then be compared in terms of the minimal initial start time difference between trains and the maximal capacity. 5 Signalling systems in OOHAA Due to the lack of the standardisation in terminology in literature within this field, this section will define explicitly the signal shadow (SS) and the distance headway margin (HM) for different signalling systems in OOHAA. The abbreviations of the remaining terms used infiguresare: BD - the braking distance of the following train; SM - the safety margin; BDi - the braking distance of the leading train;

5 - the braking distance of the following train; 5.1 Intermittent signalling Intermittent signalling means that the trains get the signalling information only at discrete points. The program defines the block boundaries at the positions where optical signals are placed. In this system the signal shadow must include at least one safety block. The number of safety blocks may be defined during the simulation set up. In this way, different multiple-aspect fixed block signalling systems may be simulated. The signal shadow is expressed in meters and defined as follows: signal shadow INT = occupied block length + safety block(s) length The distance headway margin calculated for the following train always ends at a block boundary. The figures below show the principle of the Intermittent System with one safety block, that is a three aspect signalling system. 0 iq. HM SS Figure 2: Signal shadow and distance headway margin for Intermittent Signalling System, the leading train passing a block boundary. HM SS Figure 3: Signal shadow and distance headway margin for Intermittent Signalling System, the leading train completely within a block section. 5.2 Continuous signalling - Quasi-Moving Block In this system the signal shadow is defined as follows: signal shadow QMB = occupied block length + safety block(s) length + + braking distance from the instantaneous speed for train 2 Although unnecessary, the signal shadow in this system may include safety block(s) predefined at the simulation set up. The distance headway margin calculated for the following train does not have to end at a block boundary. Alternatively, it may be situated at any point along the track. The track is still divided into blocks of fixed lengths. This intermittent system with continuous infill has the properties of a Distance to Go With Continuous Transmission Signalling System, specified as ETCS level 2, [1]. Figures 4 and 5 show the principle of the Quasi-Moving Block System without safety blocks.

6 0 io SS BD Figure 4: Signal shadow and distance headway margin for Quasi-Moving Block Signalling System, the leading train passing a block boundary. HM BD Figure 5: Signal shadow and distance headway margin for Quasi-Moving Block Signalling System, the leading train within a block section. 5.3 Continuous signalling - Moving Block The signal shadow definition is: signal shadow MB = safety margin length + + braking distance from the top speed for train 2 This system has no fixed blocks. The signal shadow has a fixed length and includes a safety margin greater than or equal to zero. The trains are informed of the target point position and report their position by radio. The distance headway margin calculated for the following train ends at any point along the track. Figure 6 shows the principle of the Moving Block System with a predefined positive safety margin. HM SS O O " ^ O O BD SM Figure 6: Signal shadow and distance headway margin for Moving and Floating Block Signalling Systems. 5.4 Continuous signalling - Floating Block The signal shadow is defined as follows: signal shadowpb ~ safety margin + + braking distance from the instantaneous speed for train 2 The only difference between the Floating and Moving Block systems is how the braking distance for the following train is calculated. The capacity of Floating Block Signalling System is improved, because the signal shadow corresponds to the instantaneous speed of the train and is therefore variable during the run. This system has the properties of signalling system specified as ETCS level 3, [1]. The principle of this system with a predefined safety margin is illustrated in Figure 6.

7 Computers in Railways Continuous signalling - Extreme Floating Block 877 As before, the safety margin length is equal to or greater than zero and is predefined at the simulation set up. Compared with the Floating Block, the signal shadow is shortened by the braking distance from the instantaneous speed of the leading train. The assumption is made that the leading train never stops instantly, for example when it derails. This approach, known as relative braking calculation, is not fail-safe, but is of interest in China. The headway margin for the second train ends at any point on the track. The signal shadow is defined as follows: signal shadow EFB = safety margin + + braking distance from the instantaneous speed for train braking distance from the instantaneous speed for train 1 HM SS SM Figure 7; Signal shadow and distance headway margin for Extreme Floating Block Signalling Systems. 6 Track and train data Data from the track data base consists of information about the position and size of stations and signals as well as the topology of track (slopes and curves). The length of the single track line is km, number of stations: 15. The average distance between stations is 9.2 km. The average block length is 800 m. The train data consists of information about the weight and length of the train, its acceleration and deceleration rates and motor performance. In order to take into account the characteristics of locomotives and their real performance, the calculation of the train acceleration depends on the tractive effort and train resistance. The influence of track topology is considered as well. Table 1: Characteristics of simulated trains. Characteristic /Train type Length (m) Weight loaded (tonnes) Max operating speed (km/h) Retardation (m/s^) Passenger Train Freight Train

8 878 1 Simulation results and discussion An example of the results obtained for the freight train running without halt is presented in Table 2, where each row includes the result of a single simulation run. The boldface rows correspond to optimal operation, because the initial time differences are as small as possible without causing intervention by the signalling system. Take for example thefirstrow dealing with the Intermittent Signalling System. The second train leaves the start station 520 seconds after thefirsttrain departures. During the entire journey, the second train never comes closer to the leading train than 144 meters, which happens at the position m. The minimal calculated headway margin in terms of time is 17.4 seconds and occurs at the same position. In order to increase the line capacity and still avoid a restrictive message by the signalling system, the minimal time difference between trains at the start station may be decreased to 510 seconds for the Intermittent Signalling System. Time gaps of 500 seconds and less cause interactions between trains and intervention by the signalling system, which is indicated by negative values of distance and time headways. The Quasi-Moving Block Signalling improves the capacity compared to the Intermittent Signalling by shortening the time difference, 510 and 370 respectively, by 140 seconds. The initial start time difference may be additionally decreased by 50 seconds for the Moving Block Signalling. The Floating Block and Extreme Floating Block allow start time intervals of 170 and 160 seconds respectively. The best results are obtained for the Extreme Floating Block Signalling. Table 2: Simulation results for the freight train, non-stop operation. Initial Time difference (s) Calculated min time headway margin THM (s) Position of min THM (m) Intermittent Signalling Calculated min distance headway margin HM (m) Quasi-Moving Block Signalling ' Position of min HM (m)

9 Moving Block Signalling Floating Block Signalling Extreme Floating Block Signalling The initial time difference In order to gauge the performance of different signalling systems, the optimal time differences between trains starting from the same point are compared Results for passenger train Table 3: The minimal time difference (s) at the start point between trains for the different signalling systems and scenarios. Results are for the passenger train. Signalling System / Scenario Intermittent Quasi-Moving Block Moving Block Floating Block Extreme Floating Block non-stop one stop two stops Each station dwell time of passenger train is 2 minutes. For operation with stops at stations, the influence of the number of station stops on the simulation results is negligible. The Intermittent and the Quasi-Moving Block Signalling Systems show similar performance in all scenarios. Floating Block improves upon Intermittent for all scenarios with reduced minimal time difference between 85 and 90 seconds.

10 880, C.A. Brebbia J.Allan, R.J. Hill, G. Sciutto & S. Sone (Editors) Results for the freight train Table 4: The minimal time difference (s) at the start point between trains for the different signalling systems and scenarios. Results are for the freight train. Signalling System / Scenario Intermittent Quasi-Moving Block Moving Block Floating Block Extreme Floating Block non-stop one stop two stops Each station dwell time of freight train is 4 minutes. For operation with stops at stations, the influence of the number of station stops on the simulation results is biggest for the Intermittent SS. The Intermittent and the Quasi-Moving Block SS show similar performance at scenarios with station stops, but at non-stop operation thequasi-moving Block SS shows much better performance compared to the Intermittent SS. The results of Floating Block and Extreme Floating Block SS performances are very similar in all scenarios. The benefits of improving signalling systems from the Intermittent to the Floating Block are biggest for nonstop operation, when the initial time difference may be decreased by 340 seconds. In scenarios with one and two station stops, Floating Block improves upon Intermittent with reduced minimal time difference of 75 and 120 seconds respectively. 7.2 The capacity The results presented above can also be expressed in terms of the capacity. The capacity of a railway line is defined in this paper as the reciprocal of the minimal initial time difference, for which the following train does not experience the restrictive message from the signalling system. The capacity is expressed in trains per hour (tph) Results for passenger train Table 5: The maximal capacity (tph) for the different signalling systems and scenarios. Results are for the passenger train. Signalling System / Scenario Intermittent Quasi-Moving Block Moving Block Floating Block Extreme Floating Block non-stop one stop two stops

11 7.2.2 Results for freight train Table 6: The maximal capacity (tph) for the different signalling systems and scenarios. Results are for the freight train. 881 Signalling System / Scenario Intermittent Quasi-Moving Block Moving Block Floating Block Extreme Floating Block non-stop one stop two stops Conclusions The simulations show the positions of critical capacity points. For non-stop train operation with both types of trains, the minimal headway occurs when the leading train has to climb a steep hill. The line capacity is then constrained by the train's engine performance and determined by the track topology. For scenarios with station stops, again for both types of trains, the minimal headway usually occurs at the station entrances, when the following train is to arrive at a station after the departure of the leading train. The disadvantages of relaying on the fixed infrastructure of the block layouts are greatest for scenarios with non-stop operation, while the capacity benefits of continuous signalling systems for scenarios with station stops are small. In order to investigate the influence of the signalling systems on the capacity of a railway network and study the effects on traffic, the development or application of a more complicated simulation tool is necessary. Such a tool would facilitate simulation of signalling system functions and the exchange of control commands. In order to study realistic scenarios it is necessary to handle meetings and overtakings of trains. Furthermore it is also important to study simulations including stochastic disturbances of the train traffic. 9 Acknowledgements I acknowledge the support of Adtranz Sweden, Banverket (Swedish Rail Administration) and Northern Jiaotong University, China in this work. 10 References [\}ETCS, System Requirements Specification, A200/SRS.04-A5499B, UIC/ERRI, Utrecht, Holland, [2] Gill, DC & Goodman, C.J., Computer based optimisation techniques for mass transit railway signalling design, BEE Proc., Vol 139, May 1992.

12 [3]Gill, DC & Sadler, S.J., Simulation Analysis of Transmission-based Signalling Systems for Metro Applications, Computational Mechanics Publications, Southampton and Boston, [4] Hill, R. J. & Bond, L.J., Modelling Moving-block Railway Signalling Systems Using Discrete-Event Simulation, IEEE/ASME Joint Railroad Conference, [5] Hill, R.J. & Yates, T.K., Modelling Railway Block Signalling Systems Using Discrete-Event Simulation, ffiee/asme Joint Railroad Conference, [6] Holgate, D. & Lawrence, P., 77?e Relative Performance Benefits of Fixed and Moving Block, World Congress on Railway Research, Florence, [7]Pascoe, R.D., Rossi, C, Savio, S. & Scuitto, G, Comparison between Fixed and Moving Block Signalling Systems Performance by Digital Simulation, Computational Mechanics Publications, Southampton and Boston, 1992.