The thing with these signals is they use the center rail to control the red and green signals. the yellow is on a timer circuit after the red has been triggered and is switched back to green. The timer circuit turns on the yellow and after a certain time switches to green. it was designed for a IR sensor. I use a automotive 12vdc SPDT relay due to I'm a automotive technician and have a large collection of relays. I use a 4amp bridge rectifier to power the relay. with the relay off, the circuit is switched to turn on the green. When the train runs through the isolated track, this supplies voltage connection to the relay and switches the contacts to the red signal. after the train passes, the relay switches back to green circuit but the yellow lights come on with the timer circuit and then green. It's very simple. wire one from transformer hot to the AC terminal of the rectifier and the other coming from the isolated track to the other AC terminal of the rectifier. DC+ and DC- of the rectifier goes to the coil of the relay(doesn't matter where + or - is wired on the relay coil as it is not specific how it is wired. the coil will work either way).
The only thing with this circuit is you will get flickering from dirty wheels or traction tires on locos. Also run the signals on aux power hookup at 12 volts. If you wire it to track power, the voltage could be too low to operate the relay.
Relays can be purchased from auto parts store and bridge rectifier can be purchased from Radio Shack. If you need a diagram I can draw one up. Very simple.
