The warning came from the union representing Queensland Rail onboard guards, who will see their guard cabins in the rear carriage of the new trains coming to the state's tracks.

The current setup allows the guards to occupy drivers' cabins in the middle of the trains, because two three-carriage units are coupled together.

However the 75 new Bombardier trains, which will be rolled out between late 2015 and 2020, will come in units of six.

Queensland Rail said this meant guards would be shifted to the rear drivers' cabin.

Rail, Train and Bus Union Queensland secretary Owen Doogan said the move was not practical.

"If you've got a passenger with a wheelchair who needs assistance and is five carriages down, how will the guards know about it?" he said.

"There's significant safety reasons why they need to be in the middle. Even if you had them at the end, it's an awful long way to go from one end to the other."

The specialised guard vans normally house one guard, and contain an emergency brake, along with maintenance and communications equipment.

A Queensland Rail spokeswoman said train guards in Sydney had been moved to the rear of their new eight-carriage trains without problems.

"As the new trains will run as a six car unit, it is unnecessary to provide a drivers compartment in the middle of the train," she said.

"The new configuration maximises the space available to customers."

The spokeswoman dismissed claims by the union that the decision to remove the centrally-positioned guard cabin could cost 600 QR guards their jobs.

"No decision has been made to cut Queensland Rail guard staff," she said.

A spokeswoman for state Transport Minister Scott Emerson also said the LNP would not cut train guard jobs, and the union's claims were "scaremongering tactics".

The new Indian-built train fleet was announced in January 2014, after the Queensland government signed a $4.4 billion agreement with the Bombardier NGV Consortium.

The deal will see the company build the trains over a five-year period and maintain them for the next 32 years.

"Unlike the previous Labor government we have been able to deliver new trains for less by paying around $13.7 million per six-car set compared with Labor who paid almost twice that amount," Treasurer Tim Nicholls said last year.

But Mr Doogan accused the government of cutting corners and purchasing "throwaway trains".

"This is the first time in Queensland's history of passenger trains that they've manufactured outside Australia," he said.

"The design means that will be cheaper, but the cost of operation will be more. You've always got to run six cars, even if it's not busy."