Here is one of them:
http://www.the-gauge.com/showthread.php?t=26200
I've asked this quesiton too, and in that thread above it was suggested that snap relay's are what i need. So now i have a couple on my kid's 4x8, i've also scanned the instructions and linked it here:
So after actually putting them on my layout i've run into a couple of logistical issues.
1) It's easier to wire 2 separate single color led's than 1 multi-color led (at least in my mind)
2) think carefully about what you want to accomplish

i went overboard on my signaling hahaha ended up w/ led's everywhere had to scale it back a bit.
3) the resitors, i've read and read, printed diagram after diagram, an expert now at ohms law .. still nervouse about popping my leds from over voltage hehehe.
I've been posting photo's of my progress here:
http://www.the-gauge.com/showthread.php?t=26923
and you can view full sized images here in case you want to see how it's all wired up:
http://www.skyersfamily.com/gallery/v/Trains/4x8/
I'll be posting some more pictures tonight after i wiretie all the loose wiring.