There's quite a few pickups there that decided they should get hit by the train. I'm not putting this in the bugs section because there's really no "bug" to swat here. So what happened?

1. On the upper track, a slow rock train cleared a busy grade crossing at speed, coming from that track that goes straight off the right side of the picture.
2. This train, which had to stop in-picture to avoid plowing into the side of the rock train, resumed- but didn't reserve the crossing until after the rock train had cleared it.
3. In that split-second of crossing non-reservation, a handful of trucks decided that since the gates were rising, they should cross the tracks. Real drivers would probably have noticed the second train already accelerating towards the crossing and elected not to get in its path.
4. Unfortunately, their crossing time is way longer than the time it took this train to actually reach the crossing!
5. No, train-truck collisions aren't simulated, so vehicles- or drivers- were harmed.

When trains approach this crossing at speed, there's enough time between crossing activation and train arrival for the trucks to get clear. However... if it were, say, a 10-track crossing, that wouldn't be true.

You can make something similar if you like, by making a really long grade crossing, with lots of tracks... or lots of crossing connectors. Or at least you can in Version 4.1 | v0.8.4a (b588), who knows when/if they'll find some way to stop this without making grade crossings way more computationally expensive than they need to be.

Helicopter Reporter: "Traffic on the 405 is backed up for miles. Dispute between a train and a angry truck driver caused the collision. Back to you Tom."