Many seem to pass by the sorting stations. What makes sense. That’s where they’re being tampered with, connected to brake lines, brake valves open, hand brakes possibly released.
There’s a lot of room for error there. Even just in cars left idle. If you leave air pressure trapped in the brake system and the temperature or outside air pressure changes too much, you can get a “uncontrolled release” of the brakes on some or all of your cars. An entire stack of cars can start moving on their own if the parking brakes aren’t on.
This is based on my random sampling of the FRA data. You can also infer this from the fact that there were 2,299 reported derailment incidents through 2018-2021, with $431,362,583 in total damages. That’s an average of $187,630 in “derailment” damages. As we know some were very expensive, most of them are very cheap.