I get asked for this a lot, so it's probably time to post it here.
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Thanks John. Very helpful.
Bookmarked as i have a pair of GG1's I want to do this too. Thanks!
I get quite a bit of email on this kind of question, so I figured maybe it would be something useful to post here. 
Is this running off of track AC power? Just wondering because it looks like it's "grounding" to the wheels. Or not, just noticed the direction of the diode.
Take another look at the diagram. Note that it's for TMCC command locomotives. In command mode, the lighting outputs are negative in respect to frame ground (wheels), and in conventional operation, the lighting outputs are AC. Given the fact that they can be AC, the diode protects the LED's from excessive reverse voltage.
Trust me, this works, I've wired a ton of TMCC locomotives with LED's using the same wiring scheme.
The .01uf capacitors are to provide a load so the R2LC triacs fire and light the LED's, ERR TMCC stuff already has these on board, so they're not needed if you're using ERR equipment. For upgrading Lionel stock TMCC stuff for LED's, you need to include the .01uf caps.
Notice that the pickup roller (center rail) is not connected, it's represented on the left hand connector. By the same token, the smoke output is also not connected in this wiring diagram.
Thanks for the clarification. I've got a project I'm working on that needs bi-directional headlights and bi-color markers. The electronics were originally from a 6-18952 PA-1, so there wasn't anything bi-directional about it. I'm still not sure if I can use it, as I'm not sure if the 691-PCB1-01B LCRU has outputs for directional headlights. There were 2 lights shown in the schematic (one with a red wire, and the other with orange), and I believe 2 pins which aren't connected, but none are labeled.
Well, the LCRU is a strange duck, and you may have a problem. It turns out that early versions of the LCRU didn't control the polarity of the light outputs, so they could come up positive or negative in respect to frame ground. Obviously, this screws up my carefully thought-out polarity management. The 691-PCB1-01B has the connectors and not the pins, so it may be based on the LCRU2, and that would possibly be handling the polarity better, I haven't tinkered with that board to see.
