Abstract :
Railway Signalling has a long history. In many cases, the specification for the equipment we use can be seen to have arisen from Railwaymen making mistakes. Usually, in the distant past, this was due to the driver of the train or the signaller making a simple error. The increasing sophistication of the signalling equipment has arisen from the desire to eliminate these errors, whilst at the same time there has been an increased requirement for automation. Railway Signal Engineers, both from within the Railway and from its suppliers, designed the equipment, developed the circuitry and, indeed, the logic, to prevent rail accidents. Also, as technology progressed, the opportunity was taken to incorporate increased automation, which was needed for economic reasons.It is an unfortunate fact that every Railway Signalling installation is a prototype, and we do not have the luxury of testing to destruction. The testing is therefore by iteration. No one technique or procedure guarantees that the installation is safe and fit for purpose. A series of relatively simple tests are put together to exercise the Signalling System. They work on a `safety net´ principle, whereby there are sufficient layers to prevent the `holes´ lining up. Thus even the smallest error is captured in the net.
