Network Rail says it was to blame for the derailment at Eastleigh that caused a week of chaos for tens of thousands of passengers.
The company revealed that bolts which hold the track in place sheered off under the weight of a freight train. The Government-owned company says it let passengers down and has issued a public apology.
An independent safety inquiry will now consider whether the incident could have been avoided and if any action should be taken against the company. Our Transport Correspondent Mike Pearse reports.