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.