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The track will also sit on concrete "slab track" which is durable but more expensive to buy than your more conventional ballast.

This reflects a practice honed along the entirety of the country’s 15-year-old high-speed rail network, casting slab track at temporary outposts close to the construction site.

The Swiss rail service said it took 43,800 hours of non-stop work by 125 labourers rotating in three shifts to lay the tunnel’s slab track.

It took 125 workers in three shifts round-the-clock to install the concrete slab track on which the trains will run, with some sections as deep as 1.4 miles.

But Network Rail estimates slab track is four times dearer to install than that on sleepers and ballast, and "it's hard to make a business case" for it, even taking into account maintenance savings.