Where can I get a replacement light for this train? It is the small grain-o-wheat style and not a twist socket type bulb. I think they even make an LED with the correct resistor for it. Does anyone know what resistor is needed to use an LED?
You can get a “golden white” 3-millimeter LED from Richmond Controls (http://www.richmondcontrols.com/). These simulate the color of an incandescent lamp, rather than the very bluish color of most white LEDs.
You will need to protect an LED from reverse voltage. The most effective way to do this is to put a diode in parallel with the LED, but with its cathode to the LED’s anode and vice-versa. Then put that combination in series with a ballast resistor. If your headlight is now connected across the track (between the pickup and the frame), you can put the LED-diode-resistor combination directly in place of the lamp. The resistor should be in the 1 kilohm ballpark; but you may find that a higher value is satisfactory, since white LEDs put out an awful lot of light.
I believe that your locomotive has a DC motor. If so, you could optionally connect the LED-diode-resistor combination across the motor terminals and get directional lighting.