Stretches of track in the Blue Mountains will have to be shifted slightly or replaced, and stations and overhead wires altered, to make about 75 kilometres of line between Springwood and Lithgow suitable for the state's $2.3 billion fleet of new intercity trains.
Commuters also face extended periods of track closures on a section between Newnes Junction and Lithgow, according to a government report into the work needed to bring the line up to the standard of the rest of the state's electrified rail network.
Transport authorities are making preparations for the work to begin early next year and expect it to take two years to complete.
At present, the line west beyond Springwood to Katoomba and Lithgow can handle only the state's ageing V-Set trains, which are narrower than three metres.
Labor transport spokeswoman Jodi McKay said the amount of work needed to make the line capable of handling the new trains was astonishing.
"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."