This is an example of the relay logic used to operate a railroad wayside signal. It's purpose in terms of signal logic are to provide an advanced warning to the conditions of the interlocking or junction ahead. Therefore this is intended to be an intermediate signal and not a control point.
The toggle switches act as control signals coming from the dispatcher desk. Each toggle simulates block authorization which gives the signal the appropriate aspect.
Aspects available are Restricted Proceed, (red/red/red) Approach (yellow/red/red) and Approach Slow. (yellow/red/green)
Intermediate signals are not control points and cannot give a stop indication as it is positioned within a block and between two control points. Also if desired, the middle fixed red may be omitted so the signal displays an Approach Medium (yellow/green) aspect.
The purpose for this circuit is to demonstrate the relay logic more than the actual function of the signal. I apologize for the crudeness of the circuit as I only have SPDT relays in the software. Therefore several coils are tied together to create the DPDT relays needed. Actual track circuits look nothing like this however the relay logic used here is very similar to what a signal would look like before the industry moved towards solid state.
Explanation of switches: (Left to Right)
1 - 1st Block authorization. Coming from the control point before and the interlocking that operates it. The first block switch will close to actuate the first series of relays and line up the signal.
2 - 2nd block (Jct.) authorization. Coming from the control point ahead with the next block authorized for diverging route to the Main.
