Serving prisoners are being used to carry out maintenance work on Britain's railways, Network Rail has confirmed.
Unions have raised safety fears
Union boss Bob Crow has reacted angrily, accusing the firm of deviousness and likening the practice to the US chain-gangs of the 1950s.
The RMT boss also suggested safety standards could be compromised.
But Network Rail said all workers were properly trained and vetted, adding that offenders would be doing "heavy lifting" rather than "specialist work".
A spokesman for the firm said one of the agencies that supplies workers takes part in a government scheme to help prisoners in the low-risk Category D reintegrate into society.
"Like many large employers, Network Rail uses agencies from time to time to plug gaps in its labour force.
"Anyone who works on the railway is properly accredited, trained and supervised at all times."
Mr Crow was unhappy that the union had not been informed of the practice.
"We aren't against anyone trying to get back into society from out of prison, but we do honestly believe that they should be taken on once they leave prison," he said.
"The fact is they've never ever told us that they've got these categories of people working on the railway.
"We don't know what safety standards these people have got."
Juliet Lyon, director of the Prison Reform Trust, said training prisoners with new skills would help them on their release from jail.
She said: "It makes far more sense to train and supervise prisoners to do skilled work in preparation for release than it does to turn them out at the end of their sentence jobless, homeless and unable to lead a responsible life."