The book I read to research this post was British Railway Signalling by Alan Williams et al which is an excellent book which I bought from a local secondhand bookstore. This book is probably not one that anyone is likely to see for sale so I focus on railway signalling rather than reviewing the book. Currently in Britain they are changing the old fashioned signally methods to computerized regional railway control centres like at Crewe & at Saltley near Birmingham. A lot of things like the types of signal used remains unchanged however. At one time when a train passed a signal it stayed on go until just before the next train arrived then would signal that a train had passed as that train passed. There was an accident however when due to ice the signal couldn't go back to stop so now the signal always go to stop until it needs to go to go. Black & yellow signals indicate distance and black and white signals indicate whether the way is clear or not. If the signal is in a horizontal position it's unsafe to proceed and vertical indicates it's safe. On an ordinary line a train will have to stop if a train has gone by in the last 5 minutes or slow down if in the last 10 minutes. Of course this would tend to be where routes converge. Similiarly if you are at a station the signals will go to a vertical state if the train is within 5 minutes travel time. On single stretches they used to give a driver a token to indicate he was the only user on that track and that token would be handed in at the end of the single stretch. Only 1 driver at a time would have a token. The token can take many forms but generally is shaped like a key and comes out of a special machine.