Signals are there to protect something. It might be a train or something in a siding. Home signals protect a station yard. Starting signals protect track sections ie. the track between stations. In life everything has a purpose therefore a signal must have a purpose. Weather it is on a real railway or a model it must be protecting some asset or it has no purpose.
This is why placement of signals is important. Another factor that effects signal positions is the ability of the train driver to sight the signal. Some times there are additional signals like outer home signals and advance starter signals. Distant signals are used to warn the driver of the trains approach to a stop signal on track sections between stations. They also tell the driver on approach to a station weather the train is required to stop at the station or not. This is only true with semaphore signals. Colour light signals used as approach signals are really just repeaters giving advanced warning of the state of the home signal.
The interlocking is designed to make sure the track is properly set before the relevant signal can be pulled. It also prevents points from being changed while the route is set. Interlocking bars and leavers are used to accomplish this as well as locking the points against movement. In the signal cabin beneath the floor there are cams which move with the leavers. These cams lock leavers that control opposing train movements.
The mechanism for operating the signals is by a leaver pulling a wire. A weight on the signal post sets the signal back to danger once the wire is released by the leaver. The points have metal rods and leavers connecting them to the leaver in the signal cabin. These rod are usually one inch pipe painted black. The leavers are cut from steal plate and allow the motion to turn corners.
These days everything is electric. Points have electric motors. Most signals use LED lights. There are relay rooms to allow every thing to be operated remotely. These relay rooms also house the interlocking which is electric of course. Electronics is used extensively. How else will the computers operate everything. Because of the interlocking computers can not set an opposing or dangerous movement. Logic to operate the signalling system correctly is also built into the computer’s programmes.
