So I just bought an old prewar set with a 153 locomotive.  Everything moves freely, but it appears that everything is shorted to ground.  It has the 2-position reverse switch but I've ruled that out as a problem by bypassing it and having the same issue.  Anyway, here is what I have so far:

1) All three commutator plates measure about 1-2 ohms (as I recall) between each other, and none are shorted to the armature shaft.  That should be signs of a good armature as I understand it.  Brushes are worn but serviceable and I do get good continuity measurements of an ohm or less from brush to commutator.  These are the old soldered in brushes.

2) I have rewired the engine, it's wired right, but because the wiring was so brittle, I didn't do any troubleshooting until after I rewired, so not sure if it was there originally or not.

3) The short appears to be the field winding, but the field is connected to ground at one end, so I'm not sure that is wrong.  It's just a coil of wire that runs from the brush to ground, so I would expect to read a near short from one end to chassis.  If I disconnect the field at the switch, then there are no shorts to chassis of the commutator, pickups, or brushes, but as soon as I connect the field, everything is shorted.  I have another prewar engine that I'm going to try probing to see where the two differ, but I'd love to get some thoughts from others on this.

Forgot to add that it does try to move in forward and reverse when switched as it should, but it overloads the transformer almost immediately due to the short.  Found this before I started taking resistance measurements to verify the short.

Thanks in advance!