Details of work needed to make the line compatible with the state's $2.3 billion fleet of new intercity trains was revealed last week. While a Transport for NSW report detailed much of the timeline for the modifications, it needs to seek separate approval for improvements to a section of the line that passes through what is known as the "10 tunnels deviation" between Mount Victoria and Lithgow.

Transport advocate Michael Paag said the lack of a timeframe for the 10 tunnels work “calls into doubt now the whole current service that we have between Mt Victoria and Lithgow”.

"If they get the new trains and the tunnels aren't widened, what do we get? Buses? That would be a major, major blow to the Upper Mountains, to the whole Mountains.

"A range of organisations has been working very hard to get an hourly train service to Lithgow. This has thrown that up in the air."

At present, the train line west beyond Springwood to Katoomba and Lithgow can handle only the state's ageing V-Set trains, which are narrower than three metres. That is too narrow for the Korean-built trains the government ordered last year.

Transport authorities are making preparations for the work to begin early next year and expect it to take two years to complete. The work proposed to bring the line up to standard will involve replacing or repositioning tracks along parts of the 75-kilometre rail corridor, extending platforms at Katoomba and Lithgow stations, and adjusting the overhead wiring and signalling systems.

While much of the minor work can be done during ordinary hours, a large portion of the project will have to take place during 10 scheduled track possession periods over two years.

They typically involve shutdown periods of 48 hours over a weekend.

But the report says that five of those possession periods for the line between Newnes Junction and Lithgow will have to each last for as long as 12 days.

Labor transport spokeswoman Jodi McKay said “commuters on the Blue Mountains Line will have to contend with two years of rail closures before they get a glimpse of these trains.”

"Not only will a staggering 60km of track need to be ripped up - but now we discover that overhead wiring, signals, station platforms and even canopies will all need to be altered," she said.

"Not only that but all the extra work needed - that the government didn't foresee - will mean commuters on the Blue Mountains Line will have to contend with two years of rail closures before they get a glimpse of these trains."

But Transport Minister Andrew Constance accused Labor of a "scare campaign with absolutely no substance to it", saying the government wanted to give people in the Blue Mountains a modern train line that was integrated with the rest of the network.

"Currently more than half of all our train fleet cannot service that line and only electric intercity trains [V-Sets] fit the Blue Mountains Line all the way to Lithgow," he said. "Standardising the Blue Mountains Line is a sensible approach and will help to future-proof our network."

The government is yet to reveal how much it expects the cost of bringing the Blue Mountains Line up to scratch will be. The $2.3 billion contract awarded to a UGL-led consortium to build the 512 double-deck train carriages does not include modifications to the line or a new maintenance facility at Kangy Angy on the Central Coast.

A spokesman for Transport for NSW said the modification was subject to a tender process and the estimated cost commercial-in-confidence "to ensure taxpayers receive the best value for money".

This article first appeared on the Blue Mountains Gazette website.