Complex sentence: He thought that the train would be perfectly safe because he knew how the signalling equipment of the time in that area worked; but he did not know that the train had stopped short of the track circuit, which would have reminded the signalman of its presence.
Split into simpler sentences: He thought that the train would be perfectly safe because he knew how the signalling equipment of the time in that area worked. What he did not know was that the train had stopped short of the track circuit, which would have reminded the signalman of its presence.