I bought two PECO double slip switches to enhance the look of the yard trackage because I just love their complexity. (A discussion of PECO's excellent products can be found at http://www.sodigi.com/Peco_Track.htm from whence I've borrowed the picture below. (Note: it is not actual size.)
I found that my concept of the way slip switches route trains was wrong and I couldn't find an explanation on the internet so I experimented and found that there are four possible routes* through a double slipswitch depending on the four combination of points settings. I set up my switches to be operated manually using ground throws via extended linkages. While my Insulfrog switch presented no wiring challenge, my Electrofrog switch needed, of course, some special wiring to handle its routing.
The switches serve in the entries to the yard and the yard can be powered either from the Main 1 throttle or the Yard throttle. This is handled by a DPDT switch. The yard, in its entirety, can be powered by either throttle so that it can operate independently from the main line while classifying rolling stock or as part of it when trains are pulling into or out of the yard.
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The circuit I devised to indicate on the panel which of the four routes has been selected is above. (Double click on the thumbnail to see the full sized image.) The A and B inputs are from the DPDT switch that selects the yard's power source and the C and D outputs are to the slip switch's Electrofrogs. In the photos the 25V and 3V inputs with those from the routing buttons are in the bottom lug strip. The outputs to the LEDs are to the right and left at the top (red and green wires) and the track polarity and Electrofrogs C and D outputs (brown and orange wires, resp.) are at the top middle.
For the present, all the circuit will do on my layout is to operate the indicator LEDs. (Note: I'm using single red/green LED's rather than the two per route shown in the diagram.) However, the outputs to the non-latching relays can be used to operate SwitchWitch® double coil switch machines to remotely align the slip switch to the desired routing if (and when) I automate the switch.
Added 27 March 2004: I have concluded that it is impossible for the circuit described above to function as I would wish them to do. The problem is that the total number of diodes to one red LED in each of the four possible configurations will reduce the voltage to that LED to below its threshold so that it will not light. I am in the process of devising a circuit that will use OR and NAND gates to light the correct LEDs based on the configuration of the slipswitch's points. Modifying this circuit to operate switch machines, should I ever decide to do so, will have to wait to that possible future date.
*top left to bottom right; bottom left to top right; bottom left to bottom right and top left to top right.