Yes, brushes /commutator.
In the pickup fault we suspected an intermittent high resistance spot or spots on one wheel and its pick-up (with the other pickup found to be making no contact at all). Moving the flywheel rotated the wheel so contact was re-established until the train stopped inevitably at the same point. Back of wheels cleaned solved this.
The plug /socket not making contact was due to a wrongly routed cable under tension - the action of touching the flywheel released the tension and the motor ran until it stopped (but this only happened in one direction) - took a while to figure it out. In this case touching the inaccessible motor may have had the same effect.