Why did they do it? It was desperation. The idea first occurred to a London solicitor in the early 1840s, when he found himself going wild with fury in a traffic jam (“Trains in drains!” he cried, smiting his forehead). By then the population was already 2.8 million, and there was no more space for conventional overground tracks. The congestion of carriages was unbearable; the horse dung was mounting in the streets; the economic inefficiency was growing.

London was the greatest city on earth, a centre of banking, shipping, insurance and law — as it is today — and the clerks could not get to their place of work. Then came the Tube, and effectively made the modern city. By the early 20th century, electric trains had created the suburbs, and enabled the professional classes to buy homes with gardens and live within less than an hour of their offices. The great Victorian gamble had paid off.

Investment in transport infrastructure — often led by the private sector, but invariably bailed out, one way or another, by the state — had unleashed the potential for economic growth. It is the transport that enables the housing; it is the housing that enables the concentration of skilled workers that produces economic competitiveness.

We need to remember that boldness today, and we need to recreate it urgently — for the good of the whole UK economy. It is true that we are making some overdue but still phenomenal improvements in the Tube. We will soon have air-conditioning on 40 per cent of the network; the Victoria line will next week go up to a record 33 trains per hour; we have put in such excellent new signalling on the Jubilee line that trains are now running three mph faster than they were four years ago (to pick a period entirely at random) and delays on the system are down 40 per cent on four years ago.

We have Crossrail’s excavations landmarking our streets, and by 2018 this project — the largest engineering project in Europe — will add 10 per cent to the rail capacity of London, on top of the 30 per cent expansion of the Tube. And none of it will be enough. Since I have been Mayor, the population of London has grown by a dizzying 600,000 — or so the census says — and we are expecting to go up by a million by 2025.

London in the 21st century looks like retaining the crown it achieved in 1800 — comfortably the most populous and commercially powerful city in western Europe. It is that extraordinary demographic pressure of nine million people that means we must have a neo-Victorian approach to transport infrastructure.

If HS2 comes into Euston — by whatever route — then we must have a Crossrail Two, from Hackney to Chelsea, to deal with the influx and also to cope with the pressure on the already straining commuter networks of south-west London. We will need to continue the improvements of the Tube — with new rolling stock on the Piccadilly line and upgrades of the Northern line; and every new train must be automatic rather than equipped with an old-fashioned driver’s cab.

We must remember that we will be able to plan and procure more cheaply if we have a steady and consistent stream of funding, and we need to learn the lesson of the terrible mistakes of the second half of the 20th century. Britain failed to invest in the transport needs of its capital, with the result that the system decayed, the economy stagnated, and the population actually declined. We allowed our docks to be replaced by Rotterdam, and we shambolically missed the chance to create a new hub airport to rival other EU capitals.

We can correct all those mistakes today, and in so doing we will put Britain on the path to sustained recovery, generate hundreds of thousands of jobs, and gain long-term competitive advantage. Yes, there was something nutty about the Victorians. But we need to rediscover their brilliance, their drive, and their self-belief. We should start with transport.