ERR install on a Williams brass daylight

First of all, points for actually analyzing the problem.

I have only one thought. I have done a couple of dozen ERR (including a couple of old Train America) upgrades, so I am not an expert, but I'm familiar. On some occasions I had to be careful to not mount the board in such a way that the "backside" solder joints could not ease into the brass (or steel or zinc) frame of the tender during use and vibration.

I don't know if this is relevant at all. Typically I will cement a piece of card stock or the like between the board's underside and the metal frame/body if clearance seems close. If the board needs support to reduce jiggle, I'll glue a bit of balsa under it as a support leg.

I've had a couple of these heat up and destroy themselves for no apparent reason.

I would check the motor,and wiring to it.

D500... I have had that in mind throughout the process because of the brass tender.  However, the tender shell was removed as part of the troubleshooting process and even with that, once the tether is plugged back in, the short re-appears.

Troy

Time to disconnect the Cruise Commander and bench test it.  You really only need four wires, power and motor.  If you want to program, just jumper ground to the program switch.  You need to start eliminating possibilities.

Had a newer bachmann reverse unit laying around.  Hooked it up.  Locomotive ran fine forward and back.  No sign of any issues.  That would eliminate not only the loco itself, but the tether solder points and the tether.  The board is the only thing left yes???

Well, it's the top suspect.  I never make absolute statements without testing the component.

I wanted to take a second to say that to all.  Gun, if you ever drop off of the forum, it just will never be as good!

The board is going back for warranty.