As we put our hard hats on I felt a strange tug back to the full swell of the pandemic, when everything was rubbed down and covered in astringent chemicals for reassurance. No mention now. We were headed down into the tunnels beneath London - a miniature version of The Tube, with proportions all the same, only everything a third of the size by contract - and not a peep about ventilation. Funny how things drift away.

We watch our step as we trace the tracks, we’re warned about low ceilings and we hear about a ghost, named, improbably, Carole. These tunnels were meant to be secret: they run beneath Whitechapel in the East to Paddington in the West, have done so since before the First World War. Beneath basements and shop windows and the busying of feet upon floorboards there lies a curved ceiling, stamped out by men with an enormous cookie cutter a century ago, now dormant but for people like us who have come to trace what was there before.

The Mail Rail, to use its expensive title, helped to shift millions of letters and packages between London’s sorting offices for decades. Then, one day in 2003, it stopped. Something to do with the money and men in suits. Two hundred people, many of whom had been part of this hardly known, rarely seen community of underground railwaymen, lost their jobs and left for the day never to come back.

Perhaps it’s the fact we’re here at the end of the day, or the remnants of the recent summer storm swilling beneath our feet, but it feels lonely in these tunnels. There are huge silver grates covering many of them; if there weren’t, we are told, you could follow them all the way through to West London. There’s a flood gate that nobody has ever used for fear that it would never open again. And there’s a curved ceiling joist, just out of reach, covered in international stamps.

The stamps are a residue of who was here before. I recognise some from when I collected them as a child; pastel images from the Nineties of kingfishers and wide-eyed children, some in the shape of triangles with perforated edges. South Africa, Zimbabwe, afar. These were the stamps, it is thought, that fell from their envelopes. They were stuck up here, instead. To their right, a newspaper cutting: Thierry Henri sporting the grin of a man who has made a record-breaking deal. It’s only attached at the top, so it hangs from its drawing pin, suspended in air as it is time. No breeze to ruffle it now, no trains or mail or conversation besides that of visitors.

Further down the tunnel is the top of a scruffy staircase. There’s not much made of it, and it makes it even more alluring. We learn that we are occupying a half of something, rather than a whole: these tunnels were cut out of the earth in circles. Beneath the tracks there is another curved room, its mirror image. These were the offices of the men - and it was nearly all men - who worked here. They still contain everything that was there when they left, 20 years earlier. Neatly labelled boxes of screws and scruffy trainers, yellowing papers and a desk chair filled with clutter. We’re not allowed down here, although we try, letting the shivers snake down our backs. There’s a sense that there must be some other life among all this detritus of the living.

Much is made of the fact the tunnels are still maintained. We have walked around a tiny portion of them, but pass through those silver gates and you could still get from West to East, an empty artery 90 metres below the city. The life above them changes - new people, new road surfaces, snazzy new offices for social media platforms - but down here remains the same. It’s impossible not to think about what they are waiting for. Perhaps we will all descend down here again, one day, and be grateful that the tunnels were waiting.