Circuitron makes a circuit board that will do what you are wanting to. The product number is FL2HD. I recently installed one on my layout, and was relatively simple. The retail price is $26.95, and I believe it will power at least 2 signals.
An added bonus of using a circuit board instead of the contactor is that you can place the signal on curved tracks instead of only straight tracks. You can also supply the board with constant voltage, so the bulbs burn at the same brightness regardless of how fast the train is moving.
The downside of this particular board is that it requires DC, so I had to wire a bridge rectifier ahead of it to make the AC that most of the rest of the layout uses to DC.
Hope this helps,
J White