Just got back from Welwyn North as it's quite local. It was the rear unit that had caught fire. An eyewitness said he saw flames leap across the track and around ten feet in the air as it came through the second tunnel, which also caused a north bound train to stop just short of the platform. By the time I got there all passengers had been transported by coaches and just the police, fire brigade and railway officials were at the station. The underside of the rear unit was still smoking, being watched by a fire crew. There was evidence of heat as the paint / decals on the side of the unit had blistered. A line block was placed and the unit inspected, and shortly after they powered up the front two units and it made its own way to the sidings at Welwyn Garden City so that the line can be re-opened. Sand was placed on the rails as oil / fluid was everywhere, and the station is currently closed to stopping traffic.
I'm in the process of editing the video, and will put a link / image once it's complete