18 volt leds

Well, there is no 18 volt LED, they're typically 1.5 or 3.0 volts, depending on color.  To run them on 18 volts DC, you use a resistor.  Add a diode for AC operation.

If Lionel used an LED and it's replaceable, they probably have the other required components farther downstream.

Are you replacing one or trying to add lighting?

Tring to replace the lighting in this sd80 mac it had none.. When i hooked up the lights i had from a mth board kit they where on fire fast.. I tested the board that the lights plug in to and there defently 14 volts or better ...All the lights and the board that connected to the board are not there .. It has one of those shells that had the springs that connected to the little board on the frame to transfer power to all the cab lights

So if i need a resister to drop the voltage... where do i get that at....

This would be alot easier if they had parts for it.. I could always just wait for the new sd80 mac in the new catalog to come out.  Buy all the lights from it .. The conrail there making is the same shell ...

You must use a current limiting resistor for EVERY LED application, even if you are only running them with a 3 volt source.

If you use an 18-volt DC source your LED's, you need a 1,500 to 1,800 ohm, 1/4 watt resistor in series with each LED.

If you use an 18-volt AC source, you'll need a 2.200 ohm, 1/2 watt resistor for each LED.

Faulty math there Rich, for 20ma (assuming you want full brightness), it's 50 ohms per volt of desired drop.  A white LED (3V) running on 18 volts requires a 15 volt drop, which would translate into 750 ohms of resistance.

It's certainly true that you can use more resistance for lower currents.

For 18 volts AC, you can still use the same resistance, however you must also use a diode to limit the reverse voltage to the LED.  Most LED's have a reverse voltage maximum of 5-7 volts.

I buy most of my parts from Digikey or Mouser as they generally have much better prices than local places like Radio Shack.