The following description is for British Railway Signaling. American and European signaling follow similar principles but with differences in some of the signal displays. 

Semaphore and 2 aspect signaling

Semaphore and 2 aspect signaling color 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 the other words as a train passes a signal that signal returns to red and the signal the train is approaching changes the 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 units operate the signals automatically following there 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 traveling slowly. To show the engine driver the aspect of the next signal a distant signal is used. For 2 aspect color 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 signaling

Color light signaling has several advantages over semaphore signaling. 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 signaling 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 speed.

If the line is very busy the signaling 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 signaling 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 signaling and a branch line would have 2 aspect signaling.

Signaling glossary

Block section. This is the length of track between 2 signals. To prevent collisions only one train can occupy this block section of track. 

Interlocking on railways means that the signal cannot change from danger whilst points are set in such a way as to derail trains or whilst another route is signaled in such a way as to risk a collision. This can be accomplished with our units using a terminal provided to set the signal to red.