"Actually, a 3 rail Lionel/MTH locomotive cannot just run with wires straight from the trucks..."
If it has a DC motor a three rail loco could run on DC current with just wires from the trucks to the motor. It just won't be able to use the sound, lights, or other features. Reversing would be done with the power pack switch instead of thru the sequencer. The current doesn't care if it's going thru 2 rails or 3 rails or overhead wires..DaveB