Randall Styx says:

You’ve started on the right “track”: you’ve tested the track with another loco. You’ve also tested the pads. So, you know the problem is somewhere on or in the loco. Somewhere there’s a break in the circuit. That break could be anywhere, from the pads to the wires to the connectors to the motor(s) to the windings inside the motor.

Are the pads oxidized, dirty, or corroded? If they’re clean metal, you have to go deeper. Take the shell off and apply either operating or test current directly to the contacts on the motor. If the motor runs or you get a current, the problem is between the pads and the motor. If not, the problem is inside the motor.

Examine the solder joints. Are they metallic and shiny or dull and chalky. Poor solder joints (dull and chalky) can break down, especially with movement. Revitalize any poor solder joints. (Just because it may have been soldered in the factory does not guarantee good solder joints.)

Check the wires for kinks or places where they might appear thinner. (The thin section could be from insulation that has collapsed over an empty space created when a broken wire pulled apart inside.) Move them around a bit and retest, or move them while you’re testing.

Closely examine places where wires are attached to moving parts (like the pads). While wires can bend, every time they bend they become slightly more brittle. Eventually, they will break. Yes, even copper will do this. The most likely spots are where the wire is attached to something that does not move or to where the wire is held fast by solder.