They were supposed to move the same way. There was an issue with the signaling somehow so they had red lights, but the traffic controller overrode it by telling them to go. Had both trains started moving instead of just the back one this wouldn’t have happened.

According to what I have read in Norwegian media: It was human error. The traffic controller said both should proceed at normal speed. The stationary train in the front told the controller that they were unable to start the train. But this message didn’t end up relayed to the train gunning towards it. The procedure was also that the trains should run at 40 kmh when there was the signal issue that was there. But they were told to use normal speed which is 320 kmh.