Anything getting lodged under the wheels of a train could be trouble.

• 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

The F1 racecar is shaped like a wedge, which lifted the train off the tracks and even got a little airborne.

I checked Wikipedia real quick:

The Los Angeles Metro Rail is an urban rail transit system serving Los Angeles County, California in the United States. It consists of six lines: four light rail lines (the A, C, E and K lines) and two rapid transit lines (the B and D lines)

Looking further, since this happened in Boyle Heights, and only Line E runs through there, it must have been a Kinki Sharyo P3010. The empty weight of one car is 45t. On pictures I can see there were three cars, so 140t (plus content) were pushing forward there. But only the first half in front of the articulation point of the first car derailed. Looking at the schema picture on the bottem here (PDF) it looks like there are three bogies. So I guess we can assume roughly 15t (plus content) would need to be bounced off the track.

I've seen the result of the same thing happening here in Zürich to our Bombardier Cobra light rail which weighs 39.2t. So I'm not that surprised anymore, but it is still impressive.

You don't even need a one-ton car. They have man-portable derailours that just hook onto the track.