Old trains running between Huddersfield and Sheffield are due to be removed by 2010 and replaced by a new hybrid tram and train capable of running between cities and on light track used by Sheffield's Supertram.

The trial, announced by the Government in March, would see the lighter and greener tram-trains used for at least two years to test out whether they should be rolled out across the UK.

But yesterday it emerged that negotiations to carry out a trial within Sheffield, which needs the co-operation of Supertram operator Stagecoach and the South Yorkshire Passenger Transport Executive, Supertram's owner, are still at an early stage.

Officials would not be drawn on whether Stagecoach supported the trial – which could force it to alter its timetable and lose out on ticket sales – and would only say that discussions were at "an early stage" and that it was "premature" to say the company was supportive.

Instead the consortium, made up of Northern Rail, Network Rail and the Department for Transport, said the trial would concentrate on getting the new technology accepted and operational on the UK's rail network.

It said that experience from Germany, where tram-trains are common, indicated that the major hurdle would be getting these vehicles running on the same line as heavy rail trains.

While much of the 37-mile Penistone line, from Huddersfield to Barnsley, is single track and largely unused, the tram-trains will be sharing the Barnsley to Sheffield segment with freight and commuter trains running at up to 90mph.

Only if that two-year trial is successful, officials said, would they move on to 'phase two', on the Supertram network.

But first, officials admitted, they would have to overcome other issues, such as whether to include toilets. Installing a disabled toilet on each of the three-carriage vehicles would reduce the number of seats, but a normal-sized toilet would be illegal under equality rules.

Not including a toilet would solve the problem, but would create difficulties on a potentially 70-minute journey.

Barry Graham, business development director of Northern Rail, said: "We're taking this very seriously, but it's certainly a headache for us."

As it stands the new vehicles will have fewer seats than the current Pacer trains, which have between 110 and 130 seats. The tram trains are unlikely to have more than 94 seats, but will have more standing room.

During peak periods it might be possible to harness two tram-trains together, doubling the capacity to 190 seats.

Simon Coulthard, Network Rail business manager, said that tram-trains could cut track wear, reduce expensive signalling infrastructure, free up space for more long-distance services, and significantly reduce the cost of building new stations.

They could cut up to four minutes from journey times between Barnsley and Huddersfield through improved acceleration and braking abilities.

He added: "We're doing a lot of groundbreaking work here and this is a learning process for the whole of the industry – and there's an awful lot of potential change that needs to be addressed.

"There's absolutely no doubt of the significance of this. The interest of the wider public has been impressive and exceeded our wider expectations. This could provide a crucial component to future UK transport policy."