This is an example of the relay logic used to operate a railroad wayside signal at a control point for a junction with 2 distinctive routes. The switches act as control signals coming from the dispatcher desk. One of these indicates the position of the turnout switch at the control point. The rest are block authorization which gives the signal the appropriate aspect.
Aspects available are Stop, (red/red) Approach, (yellow/red) Advance Approach, (yellow/yellow) Clear, (green/red) Medium Approach (red/yellow) and Medium Clear. (red/green)
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. Unfortunately there is not enough workspace to demonstrate or illustrate a solid state logic system of this scale.
Explanation of switches: (Left to Right)
1 - 1st Block authorization. Coming from the dispatcher and allowed through electronic interlocking the first block switch will close to actuate the first series of relays and line up the signal depending on switch position.
2 - Switch Position. The turnout switch at the junction will either be lined for a main line movement or into a divergence. If there is an error the interlocking will not open. Otherwise when this switch is closed it will actuate the series of relays to put up a display for main line movement. If the switch is open when the 1st block is authorized it will put up a display for the diverging route.
3 - 2nd block main route authorization. This switch will close when authorized for proceeding further down the main line. And line up the signal to show an aspect for 2 blocks down the main.
4 - 2nd block divergence route main authorization. When this switch is closed it will line up the signal to display an aspect that expresses a divergence and then clear for two blocks.
5 - 3rd block main route authorization. In most situations a 3-block system is used to express an all-clear. Anything less than a "clear" signal will be expressed with 2 or less blocks. When this switch is closed it will line up the signal to show an all clear on the main line.
