General models did directional lighting in the 50's with incandescent bulbs. If one side of the field is grounded, which Lionel tends to do, then one motor brush will have a higher voltage relative to ground than the other brush. In the opposite direction, the other brush will be higher. You can use that as a basis for directional capability.
Having said that, since LEDs run on low voltage, typically 3 volts or so for white, this system may not work as well as it did for incandescent bulbs.
Rather than use a simple resistor for an LED, there are two terminal current regulators that look like transistors. They will light your LED properly regardless of track voltage.
I am not talented enough to draw pictures on a keyboard, but you can hook them up like this:
brush 1--- diode--- two pin regulator--- LED 1--- LED 2--- frame/ground
Do the same with brush two:
brush 2--- diode--- two pin regulator--- LED 1--- LED 2--- frame/ground
That should do what you want as simply as possible. The two pin 20ma regulators are available from Digikey and other online suppliers. The diode can be any 1N400X series and it is there to prevent back voltage on the LEDs.