This image from KNBC.com (more images with the story) shows the two roller coaster trains together. The Purple train ran into the back of the red train that sits to the right of the image. They are in a braking zone. The railing on the right of the track is an emergency unloading zone if a train has to stop in a braking area.
From what I can tell this is the next to last braking zone before the train enters the station. Trains entering this braking zone would not generally come to a full stop. But they would if there was a train ahead of them waiting to enter the station.
Firefighters had to use a cherry picker to help some guests evacuate as the rear cars of the second train was stopped outside the emergency unloading zone.
Update: Reports now say that the collision did not take place at a high rate of speed. Which would indicate that the purple train had slowed down some in the braking zone before hitting the red train. The cars feature over-the-head restraints, which I hope would reduce any whiplash type injuries.