THE NSW Government has an embarrassing problem with $2 billion worth of new trains that are on order — they’re too wide to go through the tunnels.

Whereas the current trains are 2.9m wide, the new models being built in South Korea are 20cm wider. That small difference could have a big impact.

It means the new trains could collide with the tunnel walls on their way up to the world famous Blue Mountains.

But Transport for NSW (TfNSW), the Government body that manages the state’s rail system, has come up with a cunning plan. It has proposed simply relaxing current safety standards. In addition, 10 tunnels built in the 1900s will be partially modified to allow the new trains to run.

State opposition leader Luke Foley has said the government needs to show how they will guarantee passenger safety when the new trains are introduced from 2019.

“It takes a special type of incompetence to buy trains that don’t fit through the tunnels,” Mr Foley said.

Transport bosses have insisted the trains will operate safely but conceded that, in places, rules will have to be broken.

“There are parts of the network where it is not possible to fully comply with the modern standards due to physical constraints and in these circumstances, additional measures such as speed restrictions, varied track maintenance and timetabling are implemented to ensure safety,” a TfNSW spokesman said.

It’s not the first time regional trains have caused pain. In early 2016, many of Victoria’s V-Line trains were withdrawn from service after some failed to trigger boom gates at level crossings.

Regional train services around Sydney, branded as Intercity, are operated by a mixture of trains. The oldest of these, called V-sets, date from the 1970s and ’80s and operate on the Blue Mountains line. Along with XPT longer distance trains, V-sets are narrower than more modern rolling stock and are the only ones allowed through the 10 tunnels on the route.

In service for four decades, the now geriatric trains need replacing. The 512 new carriages, which will cost $2.3 billion to build, will have mobile phone charging points, accessible toilets and more space for bike racks and luggage. But they are also 3.1m wide, around 20cm broader than the V-sets.

TfNSW sets out a “kinematic envelope”, the minimum clearance around trains, which takes into account how much carriages can sway and tilt. This envelope demands a 200mm distance between the carriage and any tunnel walls or line side equipment.

But the new trains’ extra bulk will infringe these minimum distances and could see the roof and base of the trains come into contact with the tunnel walls.

In a Review of Environmental Factors report, TfNSW said widening all of the tunnels and realigning the track was prohibitively expensive but doing nothing was also not an option.

Instead, they recommended a “sub-medium electric standard” which will essentially see the current regulations watered down so the wider trains can operate.

“This option would allow the New Intercity Fleet to operate on both lines and pass each other, and therefore ensure better longer term operational outcomes, while also minimising heritage impacts through reduced tunnel lining modifications,” the report states.

In addition, the tunnels would also be “notched” in places. This involves gouging a chunk out of the existing tunnel where the clearance is narrowest to allow the new trains the pass through.

This gouge could be almost 13cm deep, much of which will take place on curves where trains are more prone to swaying.

Around a third of the total length of the tunnels will have to be modified in a process that could take two years and will involve parts of the line to be closed for periods.

Labor has said the admission that 10 tunnels will have to be modified because the new trains are too wide is an embarrassment for Transport Minister Andrew Constance.

He has had a torrid few months with a new timetable causing chaos, bruising encounters with unions and the Ferry McFerryface debacle over the botched naming of a new boat.

“Andrew Constance’s ministerial performance is a joke, but this latest bungle is no laughing matter,” said Mr Foley.

“People are right to be suspicious that Constance is going to change the safety standard to fix his bungle.”

Mr Foley said the Government should reveal the cost of the work needed to modify the tunnels and whether trains will have to slow down to get through them.

The cost of the tunnel modifications will be in addition to the $2.3 billion cost of the new trains.

TfNSW would not reveal to news.com.au the cost of the works. Nor did it deny trains might have to slow down when entering the tunnels.

“We always knew work would be necessary to deliver new trains for Intercity customers on physically constrained parts of the rail network,” a spokesman said.

“Parts of our network are over 150 years old and upgrades are necessary to ensure it can accommodate the requirements of modern trains that will benefit our customers.”

In Fairfax Media, Mr Constance was quoted as saying parts of the Blue Mountains route were 150 years old and needed an upgrade.

“I can’t understand why Luke Foley and the Labor Party don’t want a modern track and train upgrade for the people of the Blue Mountains,” he said.

In its report, TfNSW said the more modern tracks used on the line means trains are “typically more stable” so should tilt less. The Government body said notching wasn’t new, and the tunnels had to be modified in the 1970s to accommodate the V-sets which were wider than the trains they replaced.