An independent inquiry is to be held into allegations of financial abuse and impropriety at Network Rail.
The announcement follows a long-running campaign by the rail union TSSA, which had alleged misuse of public money and the mistreatment of staff at the firm.
Allegations have also been made in Parliament about "a culture of fear and bullying" at Network Rail that had led to long-term staff being forced out.
The inquiry, led by Antony White, will begin work in January.
The terms of reference for the investigation were agreed in talks facilitated by the TUC between the chairman of Network Rail, Rick Haythornwaite, and the general secretary of TSSA, Gerry Doherty.
Mr Haythornwaite said: "We have already carried extensive investigations into the allegations and found no evidence of wrongdoing.
"This further inquiry must first flush out all allegations and evidence if we are to determine whether there is a case to be answered."
Mr Doherty said: "These allegations of impropriety have been a black cloud hanging over Network Rail for the past two years. Now at last they can be examined independently and dealt with accordingly."
The Labour MP Jim Devine claimed in the Commons last year that the Network Rail head of human resources, Peter Bennett presided over "a culture of fear and bullying".
He said: "Long-term staff are being forced out, but only after they have signed confidentiality clauses that prevent the culture of fear from being exposed in the public domain.
"There is a saying among Network Rail's staff that if someone is called to meet a senior manager, they are 'away to a brown envelope meeting'."
He also claimed the payoffs could run into "hundreds of thousands of pounds".
The former chairman of Network Rail, Iain Coucher, had been urged by Transport Secretary Philip Hammond to give up his annual bonus on the grounds that he was "already handsomely paid".
Mr Coucher earned a basic salary of £613,000 and received further bonuses which doubled his pay to £1.2m. He quit in June, saying it was a good time to move.
A Network Rail spokesman said it had already held one investigation into the claims made by TSSA, but had agreed to pay for the new inquiry to "blow the clouds away".
"We hope the serious allegations made will be found to be unfounded, and if that happens, there will be a statement of exoneration," he said.