FOr DCC or DC? FOr DCC, 1K resistors. FOr DC, not quite so simple as you have to bypass the factory diode matrix and any resistor is a heavy compromise, either it doesn't light up untilt he loco is moving or it drives too much current if you run full throttle. You can do some more complex stuff to make it work reasonably well in DC but it's not a simple matter of removing the bulb and replacing it with an LED.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
I have a bunch of Proto GP7's that I would like to replace the factory bulbs with LEDS. Any idea of what LED and resistor combo I would need?