I've fixed the Bachmann "Voyager" today: so we now have a near completely working set of locos.
The fault was: as soon as it started to move, the track power was shorted and the PM42 isolated the track section. I'd removed the power car, replaced the DCC decoder with the original "DC" plug, then powered it from a 12v supply. The motor ran slowly, but took 2A. After a while it sun up to full speed and only took 150mA, and all was well for a month; then it did it again.
This time I removed the bogies to eliminate a mechanical load as the cause. Still it took 2A. I stripped down the chassis and removed the motor completely. It still took 2A, then spun up to full speed again - so definitely the problem was in the motor.
BR Lines were able to supply me a new motor, but then everything went wrong....
The new motor was 1mm shorter than the last one; that meand that one of the drive shafts didn't mate with the motor and a bogie wasn't driven. It looks like the flywheel has been pressed too far onto the axle. No problem - the plastic insert could be pulled out 1mm so the drive shaft could still be driven.
The old motor had a piece of foam around half of it. The purpose of that foam is to make sure the PCB can't short to the motor body. Unfortunately I didn't note which side of the motor it was on, and assembled the model with the motor upside down. Result - loco runs forward, with red light lit; white light for reverse.
I was able to use 12V DC with little wires to contact the motor and test it. After installing the PCB I did the same test, and the motor appeared to run OK.
The phosphor bronze strips that contact the motor from the PCB were slightly bent, and could short to the chassis. So when it was all re-assembled the forward/reverse LEDs worked OK but the motor wouldn't run. I believe it had a motor shorted to the rails. I assumed the problem was the previous short having caused the decoder to fail, so I fetched another.... similar result, this time with a burn mark on the decoder's heat shrink.
The ESU decoder tester was invaluable at this point: essentially a "known good loco" into which you plug any decoder. Unfortunately they hadn't marked pin 1 (they'd helpfully written "yellow" on what turns out to be pin 6). After resolving that the new, cheap decoder was definitely failed in one direction (it still ran in the other) but the Digitrax one was OK. Clearly the Digitrax ones are more tolerant of operator abuse.
Finally, with the motor contacts clear of the chassis and a good decoder in place, I was able to get the loco back on the track. That took an hour and a half, to do a 10 minute job!
Lessons: there's value in things like decoder testers; but getting the basics right is fundamental.