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This is an example of the relay logic used to operate a railroad wayside signal at a control point for a junction with 4 distinctive route alternatives. 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/red) Approach, (yellow/red/red) Approach Medium, (yellow/green/red) Advance Approach, (yellow/yellow/red) Clear, (green/red/red) Medium Approach (red/yellow/red) Medium Clear (reg/green/red) and Medium Approach Medium. (red/yellow/flashing 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 and 3PDT 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 thru electronic interlocking the first block switch will close to actuate the first series of relays and line up the signal depending on the 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. There is another junction at this point that also uses electronic interlocking. 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. (Will override 4)
4 - 2nd block main route divergence authorization. This is connected to the same interlocking as 3, therefore only 3 or 4 should be active but not both. I suppose I could have expressed this with a SPDT toggle rather than a pair of SPST, however the signal coming from the dispatcher desk is only expressed in 1's or 0's When this switch is closed it will line up the signal to express a distant approach on the main to a divergence.
5 - 2nd block divergence route main authorization. There is another junction at this point that also uses electronic interlocking. 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. (Will override 6)
6. 2nd block divergence route divergence authorization. This is connected to the same interlocking as 5, therefore only 5 or 6 should be active but not both. Again I suppose I could have expressed this with a SPDT toggle. When this switch is closed it will line up the signal to express a divergence and distant approach to another divergence.
7. 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. I would have added a 3rd block for the divergence route if there was enough space for the extra circuitry. When this switch is closed it will line up the signal to show an all clear on the main line.
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