Model loco revisited

Why should it matter if it's a DC motor or not? A short to ground is a show-stopper.
http://en.wikipedia.org/wiki/Brushed_DC_electric_motor

"There is a second problem with this simple pole design. At the zero-torque position, both commutator brushes are touching (bridging) both commutator plates, resulting in a short-circuit. The power leads are shorted together through the commutator plates, and the coil is also short-circuited through both brushes (the coil is shorted twice, once through each brush independently). Note that this problem is independent of the non-starting problem above; even if there were a high current in the coil at this position, there would still be zero torque. The problem here is that this short uselessly consumes power without producing any motion (nor even any coil current.) In a low-current battery-powered demonstration this short-circuiting is generally not considered harmful. However, if a two-pole motor were designed to do actual work with several hundred watts of power output, this shorting could result in severe commutator overheating, brush damage, and potential welding of the brushes—if they were metallic—to the commutator. Carbon brushes, which are often used, would not weld. In any case, a short like this is very wasteful, drains batteries rapidly and, at a minimum, requires power supply components to be designed to much higher standards than would be needed just to run the motor without the shorting."

A dc motor armature, Rag, typically has several windings. One can easily understand how one might become shorted but surely not all of them simultaneously

Meantime however I've discovered a third conductor, most of which internal to the engine which I've just now learned is presently conductive to both wheels. Thus it might not be a shorted motor but an internal path. For instance each wheel must have its own axle, with the "short" caused by the two somehow coming into contact

Anyhow to learn exactly what's happened I don't look forward to disassembling the loco

Well, doesn't every locomotive need a conductor?

Rag you've made my entire day

Thanks nevertheless for the tip regarding pole design. As I understand it (correct me if I'm wrong), this kind of motor has only two brushes so necessarily at 2 positions of the armature, the brushes in effect short the winding