If you happen to bring it up in conventional mode, the lighting circuit puts out full wave AC, the diode protects the LED against reverse voltage. Lots of folks don't use them, but after losing a couple of LED's for no apparent reason, I started using the diodes, no problem after that.
I did Loctite the coupler screws in place, and I have the trucks mounted. The couplers worked from the CC-Lite, not that I expected they wouldn't. A small roadblock popped up, I thought I had the Alco PA ERR RS Commander which I intend to use for this, but I don't have one. I'll have to wait for that to come in to totally finish. I'll have everything in place and be ready to pop that in and go. I added the cab light, a 12V incandescent at floor level
I'm adding a few ounces of weight to the center section to give it as much traction as I can. I tinkered with the chassis doing that and it actually made a difference in traction, especially starting up.
I still have to figure out how to do the handrails. Several broke taking the flimsy plastic ones off, so I'm not sure how to address that. I'm thinking maybe trying to make them from brass wire.
Why do these projects always get more complicated than you figure? 
