Transport officials want to relax standards for the minimum distance allowed between trains and rail tunnel walls so that the new intercity fleet the NSW government is buying for $2.3 billion can travel all the way on the Blue Mountains Line to Lithgow.

The intercity trains being built in South Korea will be about 3.1 metres across, which will be too wide for them to travel beyond Springwood to Katoomba and Lithgow without an upgrade of the rail line.

The first of the new intercity trains will arrive in NSW next year.

While planning is under way to alter stations and shift slightly or replace sections of rail track, eight of the 10 tunnels built early last century between Newnes Junction and Zig Zag station in the Blue Mountains need to be upgraded so that the new trains can pass through them side by side.

Transport for NSW's preferred option is to scrape away at the tunnel walls to widen them and alter the minimum clearance standard to allow the new intercity trains to run on both lines and pass each other through the tunnels.