First, if you have it or can get it, look up the article I wrote about LED circuits for the September, 2008, issue of CTT, p. 62. I did, and it reminded me of a trick that should make the tender-lights project a little easier. In any case, here's my advice about how to do it:
The two red lights: Instead of the circuit I posted above, wire the two LEDs in anti-parallel. That means that the anode of each LED connects to the cathode of the other LED, so the two LEDs light alternately, and each one when conducting protects the other from harmful reverse voltage. The red-LED pair connects to a single series ballast resistor, and that's the entire red-LED circuit, which connects directly to track voltage. The resistor value can be computed conservatively as the maximum RMS track voltage multiplied by .45 and divided by the rated forward current of the LED type used. If this is too bright, the resistance can be increased to make it dimmer, but you should not use a resistor of any smaller value. The resistor's power rating should be at least the square of the maxumum RMS track voltage, divided by the resistance, but it is a good idea to use an amply higher power rating.
Then the white LED: For this, use the circuit I posted (but with only one LED of course). For the voltage value, use not the track voltage but the maximum voltage between the motor brushes. As above, the resistance should be the RMS voltage multiplied by .45 and divided by the LED's forward current rating. But, the minimum resistor power rating may be half that given above, that is, half the square of the RMS voltage, divided by the resistance. As I wrote above, this circuit should connect to the center-rail pickup and to one of the locomotive motor brushes.