By Trevor Timpson
The Thames Tunnel built by Sir Marc Brunel between 1825 and 1843 is to form part of the new £1bn East London Railway Line.
The entrance to the Brunel tunnel as it appears today
More than 160 years after it was completed, it will connect up the transport network the capital is building to prepare for the 2012 Olympics.
Running between Rotherhithe and Wapping, its quality fills modern engineers with admiration.
Brunel employed his son Isambard - aged just 19 - as resident engineer. It was their first job together, the first tunnel anywhere through soft ground under water, and the oldest tunnel on the present London Underground.
"Victorian brickwork - particularly the early brickwork - was of a tremendous standard," explains Barrie Noble, construction manager for Transport for London, who is working on the building of the new railway.
It's such an important iconic piece of engineering
The tunnel was relined in the mid-1990s as part of a compromise after a fierce disagreement between London Underground and English Heritage. Some campaigners opposed the relining. But Mr Noble says it gave the tunnel a new lease of life.
"We don't intend to do anything to interfere with the fabric of the tunnel," he says. "Brunel and his team and his workmen were excellent."
The elder Brunel invented a tunnelling shield as a way of dealing with the waterlogged soft ground beneath the Thames - a cast iron structure that moved forward as the ground was cut, with bricklayers constructing the double tunnel behind.
Seven men drowned in floods when water burst through the shield
It was, as Mr Noble says, "a long way before its time". With no giant cutting tools, it meant 36 miners, each in his own cell in the shield, removed oak planks one at a time and cut the soil behind to a depth of four inches.
Seven men drowned during the digging - workers had to deal with sudden ignitions of marsh gas and constant inflows of water, including five major floods, in the worst of which Isambard Brunel nearly died.
"Here - almost by accident - Brunel stumbled on how you build mass urban transport," says Robert Hulse, director of the Brunel Museum in the old pump engine house at Rotherhithe, which attracts 11,500 visitors a year.
Inventing mass urban transport was not Brunel's intention. Conceived in the pre-railway age, the tunnel was meant to provide a route under the river for cargoes which had been landed on the wrong bank.
Two enormous shafts 250 feet across were to be constructed at either end and horses were somehow to be persuaded to pull carts up and down spiral ramps.
In the event, there was never nearly enough money to build the giant shafts, and the smaller shafts that still exist today remained the only way in and out of the tunnel for more than 25 years.
On its first day of opening 50,000 people walked through the tunnel, which was hailed as one of the new wonders of the world, and a million visits were chalked up in the first 15 weeks.
But the novelty wore off, and after 20 years of precarious existence as an underwater shopping arcade and a venue for tightrope walkers and sword swallowers, it was finally sold in 1869 and became a railway tunnel.
The skill of the workforce and the genius of the Brunels had achieved their aim at last - the tunnel carried cargo. It was electrified and became part of the Underground in 1913.
Kingsland viaduct. The quality is incredible, says Barrie Noble (inset)
Further north on the new East London railway, Mr Noble points to more examples of the amazing quality of Victorian builders' work.
From Wapping the line will continue to Whitechapel - as the present East London line does - and then join the viaduct which formerly carried the line north from Broad Street station.
Disused since 1986, the 170-arch viaduct defied engineers' fears about its condition, says Mr Noble.
The arches - built in 1862 - have joins to the new brickwork which was added when they were widened in 1872.
"All of those joints were totally watertight. We never found any that leaked. The standard of workmanship's incredible," says Mr Noble.
He explains how in one set of arches, an attempt was made to inject grout into the brickwork to fill voids which the engineers thought must have developed.
"There was very little flow - and we were astounded because these are so old we thought they'd be falling apart. We just couldn't understand so we did intrusive investigations: we took out bricks and we actually did radar surveys."
What they found was that of five levels of brickwork only the first, which had been exposed to the elements, had deteriorated to any extent.
The existing East London line, including Brunel's tunnel, closes on the night of 22-23 December, to reopen as part of the new East London Railway from Dalston to West Croydon and Crystal Palace.
"I'm delighted and of course it immediately increases our catchment area. When we have the overground people will find it much easier to get here," says Robert Hulse of the Brunel Museum.
He takes parties of visitors through the tunnel by courtesy of the East London line managers - who turn on the lights - and drivers, who slow down the trains.
The 1990s relining imitates Brunel's original rendering
For him the relining of the 1990s has been a good thing. "I know the purists got upset about them rendering over Brunel's original brickwork - but in rendering it over they have made it visible and accessible," he says.
News that London Underground was going to spray-concrete Brunel's brickwork led to angry protests, and the listing of the tunnel on the afternoon before work was due to start in March 1995.
In the end reinforced concrete was moulded to imitate Brunel's original rendering on the tunnel and the 60 arched openings between the two tracks.
As the London overground is built, the Brunel Museum is preparing for an expansion of its own, taking in Brunel's existing shaft, which at present stands empty above the entrance to the tunnel.
How the space in the shaft will be used is yet to be worked out. It seems likely to include some kind of viewing platform to watch the trains using the 160-year-old tunnel.
"It's such an important, iconic piece of engineering... I'm sure there'll be lots of very clever engineering solutions to the problem of how we best display what we have here," says Robert Hulse.
It is a good place for clever engineering ideas.