Why assume it's the decoder? There is a wiring harness between the engine and tender, and lot
of wires stuffed up under the shell. Can you get the shell off and see if there is a broken or pinched
lead going to the motor? It could be nothing more than that. I'd start by pulling the shell,
putting it on the track, turn up the throttle and then touch meter leads to the motor leads to see if
it's even getting power... and then trace them back to the decoder to see if there is voltage on those
leads back there.
I assume that when it was returned to you, they said nothing about what they fixed. (If it's a flakey broken wire
inside a pinch in the insulation, it could have just started working again for them. And having no idea what happened,
and no way to reproduce the problem, they may have just sent it back to you, only to have it fail again
when that wire moved a little).