How safe?

In the early days of railways, there was no means of communicating between different parts of the line and trains were run on a time-interval basis. After the passage of a train, a danger signal would be shown for five minutes (say) to prevent another train following too closely. For the next five minutes a caution signal was displayed to indicate that any following train should slow down and finally a clear signal would be shown. If a train had to stop between signalling points, the guard was supposed to run back to warn the following train. (The book from which this account is taken remarks that ‘accidents were not as frequent as might be expected’.)

Gradually, as experience was gained and technology permitted, improvements were made. Points and signals were mechanically inter-locked so that the signals could only show clear if the points were correctly set. The invention of the electric telegraph allowed signalmen to communicate and led to the replacement of time-interval by space-interval working under the block system. The line was divided into blocks and when a train passed the signalman at the beginning of a block, he placed his signals at danger until informed by the signalman at the other end that the train had left the block. Since only one train can be in any one block at any one time, collisions are, in some sense, ‘impossible’.