I agree with what ^^Bob said. Check for ballast or the rubber traction tire jammed in the gears. On a multi-motored set like this, if one motor jams or loses power, the remaining motor will not move the train.
Not sure of your comfort level with repairs, and I don't know the terms of MTH's warranty. But if it were mine, I would probably check for basic mechanical problems, disconnected wires, etc., before sending it back.