Scenarios, Stories, Use Cases: Through the Systems Development Life-Cycle

CHAPTER 17

USE AND MISUSE CASES IN RAILWAY SYSTEMS

Ian Alexander¹ and Andrew Farncombe²

¹Scenario Plus, London, UK

²John Boardman Associates Limited, Southampton, UK

This chapter presents experiences with two radically different applications of scenario techniques in railway systems engineering, described separately.

A: Use Cases for Train Control Requirements Discovery

SCENARIO WORKSHOPS and interviews were used to check whether the functional requirements for a new version of a train control box were correct and complete. It quickly became clear that the described on-board functions were essentially correct, but that the requirements as written were very incomplete. The existing requirements focused on normal operations, and essentially ignored issues of testing and entry into service. By eliciting a whole-life-cycle scenario and then exploring each phase, the issues were quickly brought to the surface. These scenarios dictated the specification of a whole-journey simulator, which would otherwise have been missed, at the price of extensive delay and budget overrun. Enhancements to provide better control and easier maintenance were also agreed to be in scope.

TYPE OF PROJECT

This was a project to replace the Train Control Unit (TCU) on all the trains on a commuter railway line. Each TCU receives command codes from a safety unit. The TCU controls the speed of the train automatically. It is overridden by the safety units applying emergency braking whenever safety is threatened. ...