Workers attempt to fix Victorian tunnel below Liverpool as trains continue to pass underneath

More than 100,000 people travel on Merseyrail each weekday and yet you would be hard-pressed to find anyone who has noticed the major repairs taking place just above their heads.

Engineering work on our train network is usually met with groans from passengers, fearful of cancellations and replacement buses.

I'm fairly sure that's what would have happened the last time services were stopped as staff tried to stop leaks in an old tunnel on the Northern Line underneath Liverpool city centre.

Now the people who own the country's railway infrastructure have come up with an astute plan which means the trains can keep on running while they carry out repairs.

Helen Little, Network Rail's project supervisor, told me: "The beauty of this job is that we've been able to do that without affecting anyone's transport in and out of the city which I think is just an achievement in itself."

The lengths engineers have gone to to keep trains running under the city is impressive.

They have built a huge scaffolding deck, known affectionately as the dancefloor, so they can carry out their work as carriages pass below.

It's possible because the Victorian-built 'high-neck' tunnel was deepened during the 1970s to make way for Liverpool's new underground metro system.

It left enough space to create the gantry where staff can work safely away from the live railway.

It's costing £3.5m to repair the brick and stone tunnel roof and to stop water leaking onto tracks.

Helen said: "With every tunnel and every structure it does start to deteriorate after a certain number of years.

"So we've come in to try and prolong the lifespan of this structure."

Helen works all over the country but hails from Speke, in South Liverpool.

It means this is a special project for her.

"Of course, we're here to do a serious job but it's nice to see part of the railway network that not everyone will get to see.

"I've been on the train many times going into Central Station and you don't get to appreciate the true magnitude of the structures that we have on the railway network.

We had entered the tunnel via a purpose-made staircase in a ventilation shaft not far from the city centre's bustling Bold Street.

"I've lived here all my life and I didn't even know it was there," Helen added.

"It's only when you start to look at a project like this and you look at your access, you look at how you are you going to do it, and you find these little hidden gems."

At about 11-metres high, the tunnel is much taller than most.

Alex Treanor used lasers to create a 3D compuler model of the tunnel ahead of the work.

It was so accurate, it proved the plan for the decking platform was possible and he was able to work out exact quantities of material.

"I got every bolt," he told me, grinning.

"The trains are still able to operate in the space below and we have this loft as we're sandwiched between rail and surface level.

"This is one of my favourite projects."

Specialist laser surveys mapped every inch of the tunnel took place to create a 3D computer model ahead of the work.

Instead of calling in contractors, the team has been specially trained to use a new ram-arch system to put steel reinforcements in place.

Network Rail says that has saved thousands of pounds of taxpayers’ money.

The project is about to enter its final and most crucial phase where concrete is sprayed into the new metal cages over the next few months.

Once complete, the work should keep the tunnel dry for up to 100 years - and the passengers below should remain none the wiser.