When you find a cheap pair of fashionable shoes there is a sense of accomplishment, because you have found a bargain.

Months later when they fall apart and you are forced to spend more money in repairs or buying another, you quickly realise the cheap option is not always the best.

The New South Wales Government knows the feeling well.

It has discovered the fleet of inter-city trains it has ordered from South Korea — at the bargain price $2.3 billion — will not fit the tracks in the Blue Mountains, west of Sydney.

While the Government denies costs are blowing out, the ABC has learned the first bill from contractor Downer EDI came in at $43 million — that is just for the construction of signalling works and modifications across the rail network.

The Government had hoped the trains would prove 25 per cent cheaper than locally made trains.

The Government hoped the South Korean trains would be 25 per cent cheaper than locally made trains. ( Supplied: NSW Government )

But it must change 20 stations to accommodate the new inter-city fleet, which means cutting away at the platforms and tunnels that are too narrow and too low.

The Opposition said platforms at Blue Mountains stations, such as Linden and Warrimoo, are too short for the new trains, which would be up to 205 metres long.

Cost could rise to $3.9 billion, Auditor-General says

Opposition transport spokeswoman Jodi McKay said the Government "needs to be honest about how much this is going to cost".

"We know they have forked out $43 million — we want to know what else is to come," Ms McKay said.

Last year the NSW Auditor-General warned that the total cost of the project would rise to as much as $3.9 billion.

This would take it well above the Government's projected saving of 25 per cent.

"They have made a right mess of this," Ms McKay said.

"We said from the beginning these trains should have been manufactured in NSW, so they fit the tracks and tunnels and we don't have this excess cost."

Other trains can get to the Blue Mountains

Transport Minister Andrew Constance said the Government always knew they would need to work on the Blue Mountains line, as the trains were bought to fit the whole network.

He does not agree the cost will blow out.

"When I said the 25 per cent, that was in relation to the procurement of trains, the manufacturing of the trains," Mr Constance said.

"As part of this new inner-city fleet procurement, we're also building a maintenance facility on the Central Coast, which will provide 350 jobs."

Existing trains in the state's fleet, such as the Tangara G-sets and Oscars, are suited for several roles.

They can run on inner-city lines to underground stations, such as Wynyard, Town Hall and Martin Place, as well as outer-suburban lines to Wollongong, Springwood and Newcastle.

The Government said there are six other trains types that cannot get to the Blue Mountains.

"We have been to market, we're in a live tender situation at the moment," Mr Constance said.

"Once I sign that contract, I will be revealing the final cost."