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. 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.