What we know so far is that the crash happened between Crewe and Warrington.

The empty passenger train was heading north and had just switched from the slow line to the fast line.

This meant the Virgin express train, which could have been doing up to a 110mph had no chance of stopping, and smashed into the other train, which was blocking the line in front.

In any accident like this, there are three main possible causes.

• It could have been human error by one of the drivers, going through a red signal.
• Or it could have been a signal failure, which means that a light which should have been red was either yellow or green.
• Or there could have been a brake failure on one of the trains, which meant it could not stop.

Several investigations

Simon Montague explains what happened

Investigations have already begun to try to establish the cause. There will be an internal rail industry investigation, led by Railtrack.

That will also involve the two train operators, Virgin and First NorthWestern, with independent technical experts called in to assist.

There will also be an independent investigation by the Health and Safety Executive, which already has two inspectors on site.

They will carry out their own examination of the trains, track and signalling systems, as well as interviews with the train drivers, and they will be looking for any breaches of safety regulations.

British Transport Police is also expected to launch its own inquiry, and the police too will be examining whether there have been any breaches of railway laws and regulations.

All these investigations will take several weeks at least.