The search for a cause to the crash has centred on a set of points just outside the station.
Railtrack has confirmed that they were damaged and could have shifted while the final carriage was travelling over them, shunting it to the side and causing it to derail and crash.
Key components of the points include three stretcher bars which hold the track in place.
Two sets of bolts which held the second and third bars in position have been found undone by the trackside.
Railtrack say without the other two bars held properly the first bar may have fractured under the stress of bearing heavy loads on its own.
How the bolts came undone is not clear.