Train companies have been accused of passing on to passengers only a "tiny fraction" of the tens of millions of pounds they receive from Network Rail (NR) for disruption to services.
The Transport Salaried Staffs Association (TSSA) said rail operators were paid £184 million by NR last year for problems which led to late or cancelled trains, but gave passengers hit by disruption only a fraction of the money.
The union said it obtained figures from the Department for Transport, which showed that only five of the 23 train operators told ministers how much they gave back to passengers in 2011, and on average that amounted to £650,000 per company.
If that figure applied across all 23 firms, it would mean less than £15 million was paid out to passengers last year, the union claimed.
General secretary Manuel Cortes said: "The original train robbers got 30 years for stealing £3 million, but each year these private firms are effectively pocketing 20 or 30 times that amount which they should pass on to long-suffering passengers but don't.
"Network Rail and the private companies are operating a cash merry-go-round in which the passenger gets taken for a ride."
The union is writing to Transport Secretary Justine Greening calling on her to force the private train companies to publish how much they receive in compensation for delays outside their control, and how much they then pay out to passengers.
Mr Cortes added: "There is at least a £100 million credibility gap here that the train companies must explain to passengers. They get compensation when their trains are more than five minutes late but the poor old passenger does not receive a full refund until his or her train is two hours late and then has to jump through countless hoops to get it."
A spokesman for the Association of Train Operating Companies said: "The TSSA is being deliberately misleading. Under a system designed and regulated by the Government, Network Rail pays money to operators for problems which cause disruption to services and put people off travelling by train.
"As part of entirely separate arrangements, passenger compensation has become increasingly generous and easier to apply for, and, as a general principle, train companies will compensate anyone who has bought a ticket for a journey that is cancelled or substantially delayed."