Signalling
The following description is for British Railway Signalling. American and European signalling follow similar principles but with differences in some of the signal displays.
Semaphore and 2 aspect signalling
Semaphore and 2 aspect colour signals both indicate the same thing, either stop (red) or block ahead is clear (green). As a safety fail safe these signals generally use the rule that the signal is at stop unless it needs to be set to clear. In other words as a train passes a signal that signal returns to red and the signal the train is approaching changes to green. However the signal the train is approaching will not change to green if there is a train in the block section ahead of it or the train is to stop at a station or a point is wrongly set. Our IRDASC1, IRDASC2, IRDASC3 and SA7-SB units operate the signals automatically following these rules.
It takes a long distance for a train to stop. If the engine driver rounded a corner and saw a signal at stop he would overrun the signal unless travelling slowly. To show the engine driver the aspect of the next signal a distant signal is used. For 2 aspect colour light signals the distant signal will be green or yellow instead of green or red. (semaphore signals have a yellow instead of red arm).
3 and 4 aspect signalling.
Colour light signalling has several advantages over semaphore signalling. It is more easily seen against obstructed backgrounds or in bad weather and is more easily controlled from a long distance away. Semaphore signals rely on mechanical rodding from a signal box being pulled by a lever and there is a limit on distance due to friction.For this reason large stations often had two or more signal boxes. 4 aspect signalling has been in use since 1935 when the London North Eastern Railway had problems with the braking distance when running the "Silver Jubilee" at fast speeds.
If the line is very busy the signalling blocks need to be closer together so more trains can be on the line. The problem now arises that a fast train has insufficient distance to slow from seeing the yellow signal before it reaches the red. To overcome this a warning of a yellow ahead is given by using a double yellow signal. Unlike 2 aspect and semaphore signalling 3 and 4 aspect signals are set at clear (green) when no trains are approaching.
So 4 aspect signals would be appropriate for model of a busy mainline, for a secondary line 3 aspect signalling and a branch line would have 2 aspect signalling.
Our IRDASC4 and MAS-Sequencer-4 provide realistic operation of 3 and 4 aspect signals.