Well, if you want to use your Cab-2 only as an emergency stop and your transformer has a constant voltage output (or an unused "throttle") you can use a small control relay with your TPC to turn the track power on and off with the cab-2.
You would wire your constant voltage to the TPC input. Then you would wire the coil of the control relay to the output of the TPC. That would be all the connections for the constant voltage. For the track power you would tie your return to the outer rail just as it (most likely). The hot would go to one side of a normally open contact of the relay. The other side of the normally open contact (possibly labeled "common") would go to the center rail of the track.
When you turn on the TPC via your Cab-2, the coil on the relay will energize, closing the contact, allowing the voltage from the throttle to go to the track. Pressing halt (or otherwise turning off the TPC will open the contact, cutting power to the rails.
Kind of an expensive way to do it, but if you have the stuff already....
Hope this helps,
J White
