"It'll be closed for forty days and forty nights," quips scheme project manager Laraine Pearson - it's clearly not lost on her that this epic project is something of a biblical scheme.
The total closure of the Severn tunnel is the longest period it has ever been shut for in its 130 year history.
The works taking place inside are to get the tunnel ready for electrification, and cleaner, greener trains.
From cables to soot
For passengers, it's meaning either buses or an extra 35 minutes onboard, but for the staff working on the project, it is a 24/7 project divided into ten hour shifts.
Network Rail say that without one long closure like this, it would mean years of weekend closures.
The work didn't begin 17 days ago, but as far back as last October.
During other closures of the tunnel, Network Rail have been sending staff into the tunnel to begin preparatory works.
They had four miles of redundant telecomms cable to remove, and a huge industrial hoover was used to remove 40 tonnes of soot from the Victorian brickwork.
You can't underestimate either the size or scale of this operation.
The compound is at Filton airbase, and from the second you arrive, safety is the biggest concern. As you sign in, you're asked how far you've driven, how long it's taken, have you the right equipment.
You're clad in high-vis, goggles, gloves and steel toecap boots.
Damp, dark and cold
Everyone gets meal tickets, to keep them fed, locators, in case the worst happens and a safety briefing that if you weren't already on the edge, really could make you panic.
I've been through the tunnel countless times, but never on an open-air rig.
As you head towards the tunnel entrance, you're torn between thinking you're in a western (and heading into a runaway coal mine) and realising just where it is you're going.
As you sit and look around, you can't help turn back round to see the light at the end of the tunnel getting smaller, and smaller.
As we dismount the rig to see the work up close, you stand on the tracks that carry 200 different trains a day. I'd been warned it was damp, dark and cold inside.
Yes it was cold and dark, but nowhere near as claustrophobic as I feared.
We made it just shy of the two mile marker, stopping to watch the team at work. One worker told me if we had got to two miles, I really would have felt the cold as the huge fans which keep the tunnel ventilated are there.
Speedy and painstaking
But, I'm really surprised at how few people are actually working inside. I count maybe 12-15 on or around the rig.
The work we see is the conductor beams being attached to arms on the ceiling.
The beams are over 11m long, but made of aluminium they are light enough to be put into place with ease, and attached quickly with bolts. Network Rail have a specially designed rig which reduces the amount of lifting their staff have to do - speeding up and making the whole process safer.
In some ways it's a speedy process but in others, you can see the painstaking pace. The actual attaching of the beams takes seconds. But between each beam, the rig has to be raised, then lowered. Staff strapped into their seats, the rig moves on another 10 metres and they do it all again.
The staff have already made their way down the entire down line - from Bristol to Wales - to attach the beams.
Arms have been attached for two-and-a-half miles.
'Phenomenal' condition
While this team work on the down line, another team are going up the other side, you guessed it, the up line.
In total 1,686 of these drop tubes are being screwed into the Victorian arched ceiling.
An arm is then connected, which will attach to the conductor beam which will eventually have electricity cable held in it.
That cable will be unwound in huge 800m lengths.
You'd expect that inside this huge old tunnel there would have been plenty of unexpected finds. You'd probably also expect it to be crumbling inside, but Laraine explains that in a "phenomenal" twist, there have been no major faults.
“It’s phenomenal really. We have carried out two brickwork inspections and numerous surveys and we expected to find work to be done but there hasn’t been anything,” she said.
Network Rail say they are on course to finish this project on time, but it doesn't mean you'll be on an electric train quite yet.
There are plenty of tests still to be done, and more work to get trains mover faster in Wales.
