I have the Lionel Hogwarts Express 7-11020
http://www.lionel.com/products/harry-pottertm-hogwartstm-express-o-gauge-4-6-0-conv-loco-5972-7-11020/

I received it as a gift many years ago.  It was tested out once, then put in storage.  I brought it out for Christmas this year and had trouble getting the locomotive to run.  I ended up having to partially disassemble it and clean oil from a few spots where it was causing electrical contact issues.  (There was oil under the plate for the center rail contact rollers, and oil around some of the edges of the larger plate that is attached to).  It has been working for a few days, but I just ran it for a few minutes and it started slowing down a little (had the throttle at maybe 50-60) and then it stopped dead on the track.  I was able to get it to do a half-second spurt in reverse, but it won't move now.  I have not taken it apart again yet.

What should I check for to figure out what is happening?  I have the smoke unit switch turned off.  I have the reversing unit switch turned on.  I've cleaned the wheels and track. The transformer is working normally.  The lights and air whistle on the train all work as expected.  The grease on the drive gears looked good when I had it open.  None of the wheels appear to be physically stuck.

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!