Invitations to run the Welsh rail network are a "missed opportunity" for passengers to have their say, industry analysts have warned.
The Welsh Government has refused to make the new franchise document public, as three companies vie to run the services from next October.
Arriva Trains Wales, who run the Wales and Borders routes, withdrew from the bidding process on Monday.
The Welsh Government said making tender invites public "risks" the process.
However, the UK's Department for Transport publishes prospectuses when it is seeking bidders to run rail franchises across England.
The winner of the new 15-year franchise will be announced in December.
The firm will be responsible for the entire Welsh network, with the exception of some inter-city services, and will also launch a new integrated transport network - the South Wales Metro.
Dutch company Abellio, French organisation KeolisAmey and MTR of Hong Kong are bidding to take over from Arriva when the franchise starts in October 2018.
But industry expert Prof Stuart Cole told BBC Wales: "I don't understand why the Welsh Government hasn't published its tender documents."
The University of South Wales transport professor added: "The English Department for Transport publish their bidding documents so the public, politicians, passengers, pressure groups and commentators can comment on the proposals.
"The passengers are the people that really count and I would have thought it would have been quite useful for the Welsh Government and Transport for Wales to hear their input."
Passenger numbers have risen from 18 million in 2003 when Arriva Train Wales first started running services in Wales, to 30 million currently.
"An invitation to tender is generally a public document," said Bruce Williamson of Railfuture, which campaigns on rail-related issues.
"So franchise bidders know what they are bidding for and the passengers, the key to success for rail companies, and passenger groups can have their input on the level of service proposed. It's a bit of a missed opportunity."
The Wales and Border franchise will be one of the most heavily government subsidised in the UK - with contributions about £150m a year - and an independent passenger campaign group wants "transparency".
A Welsh Government spokesperson said the franchise process was designed to see "experts in the field deliver the best solutions for Wales - ensuring that every pound spent has a direct benefit to passengers and communities".
The government added the invite included "sensitive details" which, if released at this stage, "risks derailing the process".
"Once the service provider has been appointed, early next year, we will make further documents, including the full tender document, public," added an official.