I have put lighting in an Athearn GP38-2 using only a bright white LED and a small red LED using Optical fibers. The Headlights above the cab are 1mm fibers flared by heat. It would have been better to use a 1.5mm or 2mm fiber.  The ditch lights and truck lights are .75mm fibers. All other lights are .50mm fibers.

To install the fibers I first drill a hole for the fiber to go through, then I cut the fiber to an approximate length.  I flare the end of the fiber a pull if through the hole and glue in place using 5-minute epoxy.

Notice the step lights and the truck lights on the test track.

This picture shows the rear of the locomotive with a red marker, the rear steps are also illuminated but that does not show.  One of the challenges in using fiber optics is that the viewing angle is not very wide.  This can be over come by sanding the end of the fiber so the scratches scatter the light

The illuminating circuit is a White LED light protected by a diode and a 300-ohm ballast resistor.  The red LED has a ballast resistor of 82 ohm and is connected across the White LED.  The LED’s are powered by track voltage that varies between 0 VDC to 12 VDC.  The lights cone on at about 3 VDC.