If a signal is located in a tunnel, it is often undesirable for a train to be delayed at that signal. The signal protecting the entrance to the tunnel is held at danger until there is a signalled path from the exit of the tunnel. The tunnel in this case might not be completely clear of earlier trains for the signal at the entrance of the tunnel to show proceed, as long as the earlier train is proceeding and the signal(s) in the tunnel is (are) either automatic or have a route set and unused (or in auto working).
Last edited by GeoffM on 15/09/2016 at 03:01