A woman was killed and 22 passengers injured in the incident in Cumbria, which occurred on 23 February 2007.

RMT general secretary Bob Crow said the men were not directly involved in maintenance on points in the area of the accident and neither would face any disciplinary action.

It is understood a third man will remain under police bail until 3 November.

The union criticised the arrests, saying the two men had been living under a "shadow of suspicion" for the best part of a year. Mr Crow said the pair now deserved an apology.

He added: "Both the Rail Accident Investigation Branch interim report and Network Rail's own report have pointed clearly to management failings and lack of resources, and it is those structural failings that still need to be addressed.

"Network Rail's spending targets have been slashed by 30 per cent over the last five years and we have raised concerns about the workloads placed on individuals on a number of occasions.

"Network Rail is still dogged by inappropriate practices brought in by private contractors and there is still too much emphasis on getting things done quickly and cheaply rather than properly and safely."

He called for a joint public inquiry into the Grayrigg incident as well as the 2002 Potters Bar rail accident, with a remit to study the fragmentation of the industry.

An 84-year-old woman from Glasgow was killed when the Virgin train from London to Glasgow derailed at Grayrigg at a a speed of around 95mph.

An initial report by the RAIB found the derailment was caused by a faulty set of points. A later report by Network rail found lapses in the inspection regime, which led to the faults escaping detection.