It is important to realise that, in research, there is no one source that will answer all your questions. The OS mapping is a source, but it has to be taken in conjunction with other sources like the railway's own survey (if done, if it survives), aerial photography (English Heritage have some for English sites), signalling plans, local authority plans, deposited plans for new railways (which can show previous installations). The fun is in tracking it down and coming to an educated decision. Over to you!