Based on personal experience in a wreck that did not involve a train, here's how it will work out. Numerous insurance people and lawyers will most likely be involved. It doesn't sound fair, but that's how it works.

Metrolink will go after the person whose car it hit for whatever damages if feels are appropriate. That person will also get a ticket or equivalent drivers license points for violating the train's right of way. Basic argument: they should not have been in front of the train for any reason. In principle, they should have stopped far enough back from the gates that if anybody hit them under any scenario they would not have ended up on the tracks - we know how that works in the real world. That person (or their insurance company) might also sue Metrolink for being there - that shouldn't get very far.

The person who Metrolink goes after will in turn go after the person that hit them, asking for damages not only to themselves directly but also to reimburse whatever Metrolink gets from them. The person that hit first (the farthest back) will also get a ticket or equivalent points for violating the basic speed law (don't hit the one in front of you). Depending on speed, status of any signals, etc. other violation points might be awarded too. That person will probably also sue the person they hit for having stopped in the traffic lane, and Metrolink for being there. And so on back if there was others in the package.

Since this was in Greater LA, it's probably better than even that at least one person in the whole mess will be judgement-proof (i.e. have no available assets or insurance) which can shift the financial responsibility in sometimes surprising ways. Lawsuits are inevitable in order to settle who actually ends up paying for what.

All things considered, though, I'm glad that it ended up as a garden-variety grade crossing collision without major injury to anybody involved. Wonder where this grade crossing is on a priority list for grade separation?