Contents 1 Introduction
Effective signalling is essential to the safe and efficient operation of a railway network. It enables trains to travel at high speeds, run close together, and serve multiple destinations. Whether by mechanical semaphores, colour lights or electronic messages, signalling allows trains to move only when it is safe for them to do so. Signalling locks moveable infrastructure, such as the points that form railway junctions, before trains travel over it. Furthermore, signalling often actively prevents trains travelling further or faster than is safe and sometimes even drives the trains. At the heart of any signalling system there are one or more interlockings. These devices constrain authorisation of train movements as well as movements of the infrastructure to prevent unsafe situations arising. Solid State Interlocking (SSI) technology, developed in the UK, was one of the first computerised interlockings worldwide. The SSI devices use 2-out-of-3 redundancy to support their safety critical functionality. Programs executed on SSI processors are responsible for safe authorisation of the train and infrastructure movements.
