The photo above shows left to right a section of 3/64 heat shrink, a section of 3/32 heat shrink, next a 3mm bright white LED. This is Lionel p/n 6102045300, and a 560 OHM 1/2 watt resistor.
Above is the resistor I used. You can buy what you need at many locations and they are very inexpensive. No polarity is needed.
Above is a front MTH conversion showing the light blue and purple wires used in the front headlight. This is the original incandescent light from the upgrade kit.
This shows the finished product without the shrink to show you the path of the purple wire to the resistor then to the LED.
This photo shows the final product with heat shrink installed.
This shows before and after. You are out about 20 minutes of your time and about two bucks. If you like the LED look, anyone can do it with no special skills. The LED I used was from Lionel and the resistor from a local parts store. You can use any manufacturer of the products. I did some testing with these a long time ago with MTH R&D and the correct resistor we came up with was a 510 OHM. You can go down to that if they are available to you.
This is an MTH PS1 upgraded Hudson for Locolawyer.
The rear of the Locolawyer Hudson.