The NSW government will need to modify station platforms and rail lines in the Blue Mountains because the new fleet of intercity trains it is spending $2.3 billion on will be too wide to make trips as far as Lithgow and Katoomba.

Tender documents for the new fleet show the trains, which are to be built in South Korea, will be about 3.1 metres wide, which is the size of a medium electric rail car.

But this means they will be too wide to travel beyond Springwood to Katoomba and Lithgow on the Blue Mountains Line.

That section of line can handle only carriages narrower than three metres, such as the decades-old V-Sets that the new trains will replace.