I actually just did some troubleshooting.  Tapping on it would occasionally make it go for a second or two.  I disassembled it, cleaned a bit more oil residue, and put the body on the track to test.  There is a white wire from the circuit board that is not soldered to the motor where it should be.  It just happened to be touching it enough to run, but the vibration or heat was enough to break the connection.  I found it by gently prodding different parts to see if anything would start it up, and when I poked the capacitor on the motor, it ran.  Further investigation led to the unsoldered wire.

I took this picture the first time I had it apart.  The wire in question is on the left hand side, the bottom of the two wires on the solder point on the motor.  It "looks" attached, but it was just held in place by pressure.
[got a forum error saying the upload folder is full, so you'll sadly have to imagine what it looks like]

I've just resoldered it, and the loco now has more considerable pep.  Better than new!