I would just go to an LHS and find one that looks like will fit the bill. Then post again with the voltage and current of what you find. We can help better from there. Don't forget the power supply, and don't ask where you can find a power supply.
Springfield PA
Note, most LED's used for our hobby are rated about 2 to 3 volts at 20ma. I use a 750 ohm resistor that allows about 12.45 ma of current. More resistance equals dimmer light. Experiment.
Do some Google searching for LED info and gain some knowledge of how LED's work.
Many DCC systems are 12 to 16 volts but can vary. Read your controller manual and decoder manual that way you do not have to be led by the hand.
Richmond Controls has surface mount LED's with leads attached. Increase the resistance until you get what you want for lighting.
There are 5mm LED's with resistor attached but might be large for your loco. You can add resistance until the light is what you want. I have not seen "dim" LED's yet.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
alloboard Where online can I find a dim 12v 5mm 30mA pre wired LED for use for internal HO interior cab lighting?
Where online can I find a dim 12v 5mm 30mA pre wired LED for use for internal HO interior cab lighting?
Try Minatronics. Here's their website link.
http://www.miniatronics.com/Merchant2/merchant.mvc
Most hobby shops stock their products. They aren't cheap, if you are buying in volume. But for small quantaties, they are fine.
Engineer Jeff NS Nut Visit my layout at: http://www.thebinks.com/trains/
I'm sure Walthers has them. Also you could order a replacement for a known loco that has one. My MTH berkshire has a cab light. I'm sure you could call them and buy a cab light board. You can see it in this pic under the roof.