The first thing that comes to mind is to make sure there is nothing bent ot binding up in the drive rods or running gear. Look for anything thats bent, and perhaps unscrew the drive rods from the wheels to see if the that is the problem. Just be careful to take notes on how it goes back together. Other than that there could be something wrong internally, possibly with the gears, or maybe with the wiring to the motor. If you are comfortable with taking things apart, you could try to remove the shell to look to see if the wires to the motor have a good connection. If so, you may be able to unbolt the motor from the gear box and if you can you could see if the motor turns when the engine is set to forward and reverse. If there are good connections, and the motor doesn't turn there is likely something wrong with the LionChief boards and that will require a service from Lionel, or perhaps simply returning the engine to the place of purchase for a replacement.
The electronics can sense when the motor is stalled and will shut down the engine if there is something wrong with the motor or driveline keeping the wheels from turning, so after a second or so the power to the motor is shut off if it can't turn freely.
JGL