What Barabas said

You only need to derail ONE of the bogies/wheel sets of only one of the wagons for the train to be "derailed". (After one bogie derails, if the train keeps moving, the whole thing will get fucked up). Bogies are usually in the ends of wagons, far from the centre of mass, so if you hit them with a car you have a lot of leverage to displace them.

Trains can be intentionally derailed by the pilot as an emergency break, which makes sense if you don't have enough space for breaking conventionally and (you calculate) the thing you'll hit won't get out of the way on time.

The state of the rails can be really bad, which also helps derailments