3462 milk car problem—bad shoe?

Thanks for the suggestions! Here's what I tried, all with no success:

• Cleaned the wheels
• Cleaned the track
• Cleaned the pickup shoes where they contact the track
• Cleaned the copper springs where they contact the opposite end of the pickup shoes (alcohol and light buffing)
• Cleaned old oil from the mechanism and checked for free movement
• Replaced the wiring, which had cracked and crumbling insulation

At this point I was about to give up, when I noticed that the pickup shoes sometimes ended up tilted to one side or the other after I put the car on the track, and this seemed to have an effect on whether it worked or not.

This car has the electromagnetic coil couplers, and the electrical contact from the pickup shoe to the car is made by a single strip of copper that functions both as an electrical connector and a spring to push the shoe against the track. The part of the rivet that touches this strip is just a small ring, since the rivet is hollow.

With the shoe tilted to one side, the spring was hitting the shoe and not making contact with the rivet end. Bending the spring slightly fixed the issue. I adjusted it so that no matter how the shoe was tilted, the rivet end could still make contact with the spring.

I made a few rough drawings to help illustrate what was going on in case anyone else runs into this issue: