Coupler problem

Clint, assuming that this coupler has been working until now, something has happened to it.  First, rule out that it is firing electrically, you would hear it buzz.  If that is not it, something is either stuck or broken.  I would start by putting some graphite into the back of the knuckle and try to close it a few times.  Failing that, you will need to remove the knuckle and look at the knuckle, springs and locking pin.  Ultimately, you may need to order a replacement from Lionel.  That should have been MTH.

Replace the coupler, they're cheap.  However, I'd suggest you order the coupler from MTH for that MTH locomotive.

John, 

Is the coupler held shut by the electro magnet?  

No, the coupler is opened when the magnet retracts the latching plug.  A spring under the latching plug holds the coupler closed.  I used to have a picture of the parts laid out, but I seem to have lost track of it.