Delivering effective & affordable train protection

The design and implementation of an effective yet affordable Train Protection System should be underpinned by a robust safety assurance methodology. In the UK, once it became clear in the early nineties that the intended British Rail Automatic Train Protection (ATP) was not affordable, a project was undertaken that led to the development of the Train Protection and Warning System (TPWS). TPWS, rather than attempting to provide full protection for all situations, was focused on the statistically highest causes of train accidents n Signals Passed At Danger (SPADrs) and over-speeding. At the same time, the safety culture in the UK rail industry was maturing and the development of the TPWS product was one of the early projects to travel through the industryrs lYellow Bookr safety proving process. Very similar processes are now contained in European standards which at first sight may appear to provide projects with a daunting set of onerous requirements which threaten to result in extended project timescales and increase project cost significantly. The temptations with safety systems in the rail environment is to demand excessive safety integrity levels n often those of the primary signalling system for everything connected with train control. Whilst this produces system safety requirements which are easily defendable it often leads to overly engineered and unaffordable systems which are not implemented and therefore fail to deliver the primary intention, a safer railway. The TPWS project demonstrated that it is possible to deliver an affordable product implementation developed using the now almost universally recognised safety proving processes applied in an appropriate way. This enabled cost-effective yet robust development and proving and demonstrated that the application of such standards need not add an excessive overhead to the development life cycle. After all, unless implemented, safety systems provide no safety benefit at all. Today TPWS is a testament to the success of the lYellow Bookr process and is a clear demonstration that lsafe enoughr can be appropriate in a rail environment. The benefits of TPWS in preventing and mitigating serious accidents can clearly be seen from publicly available reports. It is also clear that delay in implementation of the system would have probably resulted in further loss of life on the UK railway. This paper describes the practical application of the processes used; the issues faced and tips for success. It uses the UK experience to provide recommendations for others faced with the challenge of implementing world-class safety standards on an existing rail network.