A 46-year-old rail worker has been questioned on suspicion of manslaughter after a train crash near Kendal in Cumbria in which an elderly woman died.
Margaret Masson died in the crash
British Transport Police (BTP) said the man was a Network Rail employee, who has since been released on bail.
The London to Glasgow Virgin Pendolino train derailed on 23 February at Grayrigg, killing Margaret Masson, 84, from Glasgow and injuring 22 others.
A subsequent investigation blamed the incident on a faulty set of points.
A BTP spokesman said the 46-year-old man, from the Preston area, was arrested on Wednesday in connection with the incident.
He has been released on police bail until 31 October.
Network Rail refused to comment on the arrest.
The initial report into the derailment from the Rail Accident Investigation Branch (RAIB), said faults with the points meant the tilting train could not follow its intended path over the tracks and it derailed.
Investigators found one of three stretcher bars was not in position, two were fractured and bolts were missing.
The bars join the moving rails, keeping them a set distance apart.
The driver of the train, Iain Black, 46, from Dumbarton, who was hailed a hero after it emerged he stayed at the controls as it careered off the rails, suffered neck and shoulder injuries.
The (RAIB) investigation said there was no evidence to indicate the driving of the train or the condition of the train were contributory factors to the derailment.
Two access roads had to be built to deal with the removal of the nine derailed carriages.