Model train incandescent (wheat grain) bulb to LED
This is a pretty simple and cheap constant current circuit:

Although LEDs are diodes they won't tolerate reverse voltage above 5V usually, likewise transistors don't tolerate reverse B-E voltage above that, so I've added a diode D2 which will conduct only when supply voltage is positive to protect the other components. For front and back light you need one circuit like this one for each light, but the one for back light should be connected to the power supply in reverse polarity so it lights when the voltage is negative.
Q1 and Q2: standard NPN transistor like PN3904, BC547, etc
D2: 1N4148, 1N4001 etc.
Current is set via R1 and is about 0.6V/R1, for 47 ohms that's 0.6V/47ohms = 12-13mA
The interesting bit about this circuit is low overhead voltage, it will only add about 1.2V on top of LED voltage.