Debris caught in drive wheel of Diesel.

Remove the shell and rotate the motor flywheel. Use a pin to carefully remove any remaining debris.

MELGAR

Remove the shell and rotate the flywheel on the motor by hand. Just back them up and clear the junk out of there. 

Put it upside down in an engine cradle apply power to it and it will fly right out hit it with an air gun-lubricate and run it. Nick

Check the grease. It can grab ballast.

PS: I use a dental pic.

I had to loosen the motor on an MTH GP40 to remove a small piece of ground foam caught in the drive gears. It wouldn’t move either way with the flywheel. With the motor loose, I could back it out with the drive wheels. If it is a dual motor, which it is, only the free motor will spin in a cradle.

You can also use a tooth brush to remove dust, ballast, etc from the drive gears.  After you get the motor turning freely, prior to putting back on the tracks, vacuum the track area.  Remove ALL loose ballast, dust, cat hairs, scenery, etc., then place the engine on the track with the shell off.  Run it.  Clean any remaining junk before installing the shell.

Stop slathering grease on the gears, it's great for picking up scenery and ballast.

I would just like to thank everyone who responded to my question and gave me lots of good advice.  Thank you.  I will work on it today.