Developing arrangements for "open access" to rail networks, together with the establishment of service reliability targets and penalty charges, requires a new understanding of both system capacity and the effect on reliability of additional services. The separation of trains on any one line depends upon an interaction between the traction, route alignment and signalling. The capacity of the whole system however, is also determined by junctions, and the structure of the intended timetable, reflecting cimmercial aspirations. However, the practical capacity limit must also take account of service reliability in the face inevitable disruptions. Service reliability is principally a function of intensity of operation, increasing the chance of "delay by reaction" as each late train potentially delays others. Conventional planning techniques are adequate to plan timetables, and suggest whether changes to the traction, route, signalling or timetable will enhance or jeopardise capacity and reliability. However, decisions involving capital investment or the balance of advantage between competing factors require quantified data on reliability. computer simulation can be used initially to refine the headways and junction clearance times on which the service is based. Then it is possible to merge data on the traction, route and signalling with a timetable plan to simulate operation of a complete service. Further, input data for the probability and magnitude of delays to trains on handover to the simulation area can be added, testing the risk that these delays will escalate "by reaction". The Comreco Rail Ltd. Railplan~TM simulation system offers detailed output on the operation of individual trains and of complete services. Comparison of initial and final delays to trains quantifies the "delay by reaction" within an area. This information can then form an input to patronage and revenue forecasting, and to investment appraisal.
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