CPN'09, Aarhus, Denmark, October 19-21, 2009 Verification of Railway Interlocking Tables using Coloured Petri Nets * Somsak Vanit-Anunchai School of Telecommunication Engineering Suranaree University of Technology Nakhon Ratchasima Thailand * Supported by National Research Council of Thailand

Introduction to railway signalling CPN'09, Aarhus, Denmark, October 19-21, T 42T 103T 103 9T 3T 1T T 16T Railway Signalling System divides rail track into sections. Only one train is allowed in one section at atime. A section or route comprises wayside equipment 1) Track Circuits used to indicate the presence of trains 2) Signals to allow the train enter into the route. 3) points (switches ) to diverge the train to another track. Each wayside equipment has an Identification number. 3

A typical (small) station CPN'09, Aarhus, Denmark, October 19-21, T 42T 103T 103 9T 3T 1T T 16T Require TC 3 1 Route 3(2) locked Route Released  Normal Interlocking Tables or Control Tables are the tabular representation specifying how the train move together with the states and actions of related equipment.

Approach Lock = Cannot cancel CPN'09, Aarhus, Denmark, October 19-21, T 42T 103T 103 9T 3T 1T T 16T Signalman can cancel

CPN'09, Aarhus, Denmark, October 19-21, 2009 Motivation (Problems) Problems with manual inspection of railway Interlocking table  labour intensive, erorr prone State Railway of Thailand’s projects involves stations Existing track layout changed (added)  existing signalling changed. Other software tools usually are designed for a specific railway company but SRT’s Operating rule is unique and sometimes changed. Need simple formal methods for signal engineers

Selected related work (quick look) CPN'09, Aarhus, Denmark, October 19-21, 2009 Logistic

CPN'09, Aarhus, Denmark, October 19-21, 2009 Selected related work

CPN'09, Aarhus, Denmark, October 19-21, 2009 Selected related work

Our CPN model of the Control Table of the small station comprises two parts 1.Signalling Layout 2.Interlockin g The CPN model comprises 72 Places, 12 Fusion places, 21 Substitution Transitions, 33 Transitions and 12 ML functions. CPN'09, Aarhus, Denmark, October 19-21, 2009

CPN model of the Control Table 1.Signalling Layout - The CPN model mimics the signalling plan - Provides geographic information how each wayside equipment connect to each other - Provides ability to simulate the trains moving - Comprises lower CPN subpages which represent the trains’ movement when passing signals, passing point and moving between 2 consecutive track circuits  modelling wayside equipments CPN'09, Aarhus, Denmark, October 19-21, 2009

CPN model: The southern part of the station

Modelling Approach The CPN model in the signalling layout part depends on the track layout.  It is inevitable.  But the CPN diagram can be quickly, manually built when we have CPN patterns (library).  The work on CPN patterns for this project is in progress. CPN'09, Aarhus, Denmark, October 19-21, 2009

CPN model of the Control Table 2. Interlocking part comprises 3 CPN subpages 2.1 UserCommand  sets and locks the points along the route 2.2 Routesetting  sets the required route 2.3 RouteReleased  using the passage of the train restores the route to Normal state and unlocks the points CPN'09, Aarhus, Denmark, October 19-21, 2009

Modelling Approach The CPN model in the Interlocking part depends on the contents in the control table. Because of 300 stations (to go), we attempt to make the generic net structure. The contents of the control table are coded in ML functions used in arc inscriptions.  Thus 300 stations can use the same net structure of the Interlocking part. CPN'09, Aarhus, Denmark, October 19-21, 2009

CPN Model: Route Setting require_point_normal(route) ++ require_point_reverse(route)

CPN'09, Aarhus, Denmark, October 19-21, 2009 Excel  XML XSLT script It took me 2- man-months to complete the first model (including analysis). But the double track station  It took me only 8-man hours to build the model (not including analysis).  ML functions are automatically created from XML control table using XSLT.

CPN'09, Aarhus, Denmark, October 19-21, 2009 CPN Model: Route Setting require_point_normal(route) ++ require_point_reverse(route) This part is a great help regardless of assumptions.

CPN'09, Aarhus, Denmark, October 19-21, 2009 Assumptions and their affects on the correctness of the model To start building the model we have 10 assumptions. Q: The important question is how these assumptions affect the model. A: I consider that there are some differences between the real system and the model. However the model in this paper can detect a large part of errors which we always encounter. A larger part = something is missing or added (extra) in the Control Table.

Analysis The desired property is no collision.  No two train in two consecutive track circuits.  Using ML query functions. To convince the model correctness  After route(s) setting and train(s) movement, The terminal markings shall be as we expect.  To debug the model using an incremental approach. Starting from one route setting - one train CPN'09, Aarhus, Denmark, October 19-21, 2009

Terminal markings CPN'09, Aarhus, Denmark, October 19-21, 2009 Using query ML and state space search No train collision is detected in case A,B and C

CPN'09, Aarhus, Denmark, October 19-21, 2009 Conclusion A control table for the small and typical single line railway station is modelled and analysed. This CPN model can be adapted and re-used for SRT’s double track projects ( stations). We propose to convert Control tables to ML functions using XSLT. Thus the CPN models of other interlocking can be rapidly built. These models will help to detect errors in control tables in the early phase of system development.

Future work Relaxes modelling assumptions Revises the CPN subpages and arranges a library of CPN patterns Create CPN models directly from Track layout drawing. CPN'09, Aarhus, Denmark, October 19-21, 2009

Thank You! Questions and comments?

CPN'09, Aarhus, Denmark, October 19-21, 2009

Initial markings CPN'09, Aarhus, Denmark, October 19-21, noTrain at other places - setting commands for all 8 routes - Both blocks in Coming states - A Block request command for going toward Bangkok

Analysis results CPN'09, Aarhus, Denmark, October 19-21, 2009 More trains  less number of possible train movements Less trains  more number of possible train movements Not true in general (e.g. double track and large stations) State space sizes