HSE Hatfield investigation team leader Frank Hyland outlined on Tuesday how it took 17 seconds for the train, carrying 170 passengers and 12 GNER staff, to come to a halt after the rail began to crack.

At about 1223GMT on 17 October, the train - made up of a locomotive, six second-class coaches, a buffet car, two first-class coaches and a rear cab - comes under a road bridge which crosses the track, south of Hatfield.

It is travelling at about 115-117mph with the speed control set at 115mph.

Just over 43 metres past the bridge, the leading wheel of the locomotive on the left hand side comes into contact with what the HSE calls the "southern most point of fracture" - where the rail begins to break.

The HSE has reconstructed the fragmented rail

The locomotive and the next two vehicles travel over the fracture point without coming to harm but the HSE can tell from damage caused to the underside of the coaches that the rail had already started to disintegrate.

The coaches, from the third coach onwards start to derail, their left-hand wheels coming off the track.

Some of the bogies (the swivelling device which holds the wheels to the carriage) shear off and several coaches begin to tilt.

Coach three tilts about 45� to the left and smashes into a signal post at the trackside.

The next coach remains upright, the coach following that starts to tilt, and the coach following that starts to lean even more.

The buffet car quickly flips onto its side and skids along, tearing up the track.

As this happens, the coupler between the buffet car and the first of the First Class coaches uncouples. The first two First Class coaches also part, causing the automatic braking system to kick in.

The train has now split in two, with both sections slowing at different rates.

The coaches tilted and skidded off the tracks

The buffet car, which is on its side, starts to swing outwards and its roof crashes into one of the posts supporting the overhead power lines.

The front section of the train continues to skid northwards and HSE experts think the buffet car crashes into another post.

The rear end of the buffet car has sustained the worst damage and this is where four people lose their lives.

Others in the front of the buffet car are badly injured.

Finally, both sections of the train come to a halt, approximately 600m from where the rail began to break.

The whole sequence of events has lasted 17 seconds.