More than 900 passengers have given money to support a legal case against the Government's handling of the Southern Railway franchise.
The Association of British Commuters has raised almost £18,000 towards a target of £25,000 to seek a judicial review of the award of a contract to Southern's owners Govia Thameslink Railway.
See also: Southern Railway passengers fund legal review
SEE ALSO: Passenger anger at 'Southern Fail'
Southern services have been disrupted for months because of a long running dispute over the role of conductors and staff shortages.
The Rail, Maritime and Transport union is planning a wave of 14 days of strikes from October to December.
Lawyers acting for the commuter group have written to the Department for Transport requesting documents about the Govia agreement.
The group claimed its requests have not been granted.
A statement said: "We have been delighted with the public response to our campaign.
"What has been extremely disappointing, however, is the Department for Transport's failure to engage with us.
"We have asked the most straightforward of questions and have been met, firstly with two weeks of silence, and now with a further delay whilst they consider whether to lift the lid on the franchise. This conduct just underlines the fact that judicial review is the only option available to us."
Emily Yates, campaign co-ordinator for the group, said: "We are very disappointed that the Department for Transport has failed to reveal documents that we feel the public urgently need to see, considering the catastrophic state of rail travel in the South.
"It is hard to understand why they cannot be transparent about their dealings with a private company that is failing on such an unprecedented scale."
Summer Dean, another spokeswoman, added: "If there's nothing to hide, why not reveal these documents, which form the area of the whole Govia fiasco that most urgently needs clarifying. We cannot see any justification for the DfT continuing to conceal this vital information, and call on all our supporters to help us in taking our application for judicial review forward."
The group is planning to stage a protest at London's Victoria station on Thursday.