The Bachmann Austerity 2-8-0 is fighting me every inch of the way.
Having established that there was a short circuit, I stripped the loco down (again) and replaced the wires from the tender plug to the pickups and the motor. Reassembled it and first of all it ran on DC, then it didn’t, then it started getting hot. It turns out that one of the three orange capacitors was just touching the top motor terminal.
So after some quick repositioning it started running on DC, and ran well into the bargain.
Time to try it with a decoder - and this is where I made the rookie error.
Flipped the track over to DCC but forgot that the loco with DC blanking plug was still on the track. The result being a fried motor. Doh!
Fortunately Bachmann have a decent spares dept. so I’m now waiting on the postman.