Mike,
You have had some good responses. Obviously if the loco is brand new, maybe a factory fix is in order. If not new, and you feel up to getting into a fix, here's where I would go: While not knowing what if any control system you are using (like DCC, battery, etc.), assuming basic principles, either the motor is getting power (and not operating), or it isn't getting any juice. The track pickup is not an issue (you are track powered? From reading your post) as the other functions are getting power. Kevin asked if you can hear the motor trying. If you cannot, then trace the power flow from AFTER the track pickup, through all the switches, to the motor.
There are several weak points to look for: All solder joints from the main PC board, through the switches, and the actual motor connections themselves. Solder joints do fail! Without knowing if your loco is new, or has been DCC'd, etc. it's hard to be more definitive. For example, if I knew that you were DCC I could rule out a bad filter board on the motor, as you could not have that there and still work in DCC. It would have to be removed at some point.
Kevin's advice on taking the loco apart is spot-on, with two exceptions: There are SIX screws (3 per side), and the one he mentioned under the smoke box. The rear screws are hidden behind the rear drivers and are difficult to see without working the suspension a bit. Further, in order to separate the cab/boiler assembly, you will need to remove the Johnson bar screw which joins the rod connected to the control lever in the cab RH side to the chassis. This is a brass screw, is difficult to see and remove, more difficult to replace, and is non-magnetic, so be careful! Unfortunately, to check your wiring, you're going to have to go through this exercise.
If I can be of further assistance, don't hesitate to PM me. I have two of these Moguls and have been all through them.
Regards,