What would bend a full-size Amtrak passenger car into, basically, an L-shape?

[Xithras]

24. Modern Amtrak train cars are built of pretty thin steel.

Many of the multiple deck passenger cars don't even have the heavy steel frames under them anymore that older passenger cars used. The design provides more passenger space and consumes less fuel, but they have all the rigidity of an aluminum soda can in an accident.

The cars are large enough that their overall weight is still high, so there's lots of potential energy being transferred in an accident. Lots of potential energy + lightweight materials = major structural damage.

As for what caused it... It's entirely possible that the train car simply dug into the dirt and came to a halt. If the dirt was soft enough to allow the car to dig in, nothing else would have been required. The momentum and potential energy of the cars still moving behind it would have been more than enough to crush it and rip it apart.