Wikipedia
SimSig is a mixed donationware and commercial Windows-based train simulator of modern railway signalling systems in Great Britain, from the point of view of a railway signaller. Users have also had success running SimSig on Linux using Wine.
The program was written in Delphi 6, a dialect of Object Pascal, by Geoff Mayo and has been in development since the late 1990s. Visually, it resembles the British Rail Integrated Electronic Control Centre (IECC), though most of the simulations do not cover areas operated by IECC-based signal boxes. It simulates overlaps, approach locking, time-of-operation point locking, shunt routes, warner routes, call-on routes, and more.
Railtrack asked for a "professional" version of SimSig, now known as TREsim, which is currently used to train signallers at every Network Rail IECC and several panel signal boxes around Great Britain.