Dark holes in Rotherhithe don’t normally strike people as a must-see. But when it’s Marc Brunel’s 1843 Thames Tunnel, open to the public possibly for the last time before the London Overground rail service takes over, then access to a cold, damp tunnel becomes the hottest ticket in town.

So hot that sadly many people were being turned away at Rotherhithe railway station and the nearby Brunel Museum – it seems a TV report this morning implied tickets were available. They weren’t – they were only available online for a brief spell last week before the London Transport Museum’s ticketing website crashed. A lucky few managed to slip into the station for the tour – the rest made do with a visit to the museum and a natter with a volunteer on the door, bemused by how chaotic the whole thing was.

Rotherhithe station’s all but ready for its rebirth as a mainline stop – the old building’s been completely refurbished, only the faded ads on the escalator looking out of place, including one for Joseph and a not-quite-so-Technicolor any more dreamcoat. Health and safety spiel done, rubber gloves on – to protect against Weil’s disease – and down to the platforms it was, and the odd experience of walking in the middle of a railway track. My initial thought was that this wouldn’t be much fun for claustrophics, but once inside the Thames Tunnel proper, the lighting and the concrete treatment applied to the tunnel emphasised what a substantial piece of work his was.

Brunel’s tunnel was the first under a river anywhere in the world. Victorians would come to fairs down here, with people selling items from the arches between the two carriageways. Even on foot in the middle of the tunnels, it’s hard to imagine stalls between these small spaces. But the tunnel would have looked vastly different then – lit by gas and reflecting off fresh, clean brickwork – and no rails in the way. It was a commercial failure, and was later sold for railway use – with Victorian steam moguls eyeing this up as part of a route to the continent – via Baker Street, the Circle Line, and New Cross. From May, it’ll be rejoined to the mainline once again, but to Croydon rather than Calais.

The brickwork had concrete applied to it in the 1990s to protect the tunnel – said to be the leakiest on the Underground network – but after a row broke out between London Transport and preservation agencies, a small part close to Rotherhithe station was left alone. Rather than a reminder of how the tunnel was, the existence of the exposed brickwork just seems to justify the decision to cover it in concrete in the first place.

We walked up to Wapping, where the tunnel entrance is at the end of the station platforms, and back down to Rotherhithe again – a strange experience in itself, to be able to walk under the Thames at this point. Indeed, the loss of the old East London Line severed a useful artery of the capital’s transport system – it’ll be good to have it back in its new guide. (The nearby Rotherhithe Tunnel is actually open to pedestrians, but I can’t imagine it’s used by more than a handful of souls.)

Back out of the tunnels, and we removed our gloves – which had become sticky and sweaty during our tour. I shared the tour with Peter Watts, who remarked on how many women there were on the tour. Old railway tunnels usually bring only a certain kind of man out – but the Thames Tunnel represents so much more, a fragment of a London which has gone forever. Tonight and tomorrow the Brunel Museum is holding “fancy fairs” – recreations of just what took place in the tunnel after it opened. For me, just that glimpse of Brunel’s lost world under the Thames was enough. Enticing as a “fancy fair” sounds – it’ll be much more popular when the railway reopens.