The next train may not be a train at all

In the 21st century will Londoners hail Zevco's yellow cabs, identical to the old black cabs, except that they fill up on water, and leave water vapour and hydrogen in their slipstream? Just one of the options in the Fast Forward>>Rewind<< exhibition on transport from 1900 to 2026 at the London Transport Museum in Covent Garden, it's an invention of the 20th century that could catch on in the next.

Like the monorail. As Richard Horden was hurtling to work at Munich University where he heads the architecture and industrial design department in an elegant air-conditioned monorail cabin, so silently and smoothly that he could finish his fineline drawings, it occurred to him that it was exactly what London needed. Without any backing, he and his partner Billie Lee set about plotting its course along the River Thames on the South Bank, from Heathrow via Syon Park and Battersea to Westminister, thence on to Terence Conran's empire at Butler's Wharf and ending up at City airport.

With 17 bridges along the way, he designed the monorail supported on slender concrete columns pounded into the Thames to run underneath the bridges, doubling as sheltered landing stages. He reckoned on it taking 1,500 people in each direction every hour. Estimated to cost pounds 32m per completed mile, the route taken would cost pounds 1bn, half the cost of Cross Rail which requires expensive tunnelling and stations.

He sent his plans to John Prescott in August 1997, hoping that the monorail would get the Deputy Prime Minister out of the traffic jam and not fudging the bad news that London's narrow medieval street network would not easily accommodate a tram network, that a river-bus system would be slow and dangerous that buses generate polluting emissions. Perhaps it was his final point, that "the underground system is too expensive to build and maintain and, I believe, unsuited to providing a fast, qualitative and `visible' transport environment for the next millennium" that provoked John Prescott to write back to express his complete lack of interest.

An architect of remarkable tenacity, Horden hasn't given up on the scheme though he can no longer afford to run his practice as a hot-house for ideas. Now his practice, Horden, Cherry, Lee, design office blocks instead and he hopes that a business consortium will rescue the plans to give London its first cheap, energy-efficient transport system. It is such a good idea that you can't imagine why it has not caught on. You can see it on exhibition at the London Transport Museum where all the architecturally designed systems - except for a plainly impractical fleet of walking cities with Dalek buildings on telescopic inline-skates by Archigram - have the disclaimer "this is unlikely to be built". That too was the fate of Chassay and Last's cable car system, which celebrates the Millennium by running a ski-lift from the masts on the Dome all the way along the Embankment. Some of the ideas in the exhibition have already come to pass in London. Projects like Termite city in Judge Dredd with overlanders moving underground through Autopsy Underpass to suffer road rage in travel tubes. Others are out of time, such as Fritz Lang's visionary film, Metropolis, from 1926, with interconnecting aerial bridges between skyscrapers and aeroplane dirigibles.Clearly not an option. To find out what Londoners and visitors want, with each ticket, London Transport Museum issue a counter to post into one of the seven long clear plastic tubes, numbered to indicate the preferred option. Typically of London Transport, these are above the reach of a child unless they climb stairs provided (which doesn't help the disabled).

There are seven options: the return of trams, inspired by Croydon's tramlink; more underground lines like the Jubilee line extension; riverboats; extra cycle lines; alternative vehicles powered by the sun or electricity, steam or gas; car-free London with controlled traffic and traffic clear zones; and more pedestrian-friendly initiatives such as wider pavements and more zebra crossings. These last two options, which depend upon each other, are the least popular options despite the Achitecture Foundation's excellent submission for "Car-Free London" which include zero-emission buses with cycle racks (good), expanding London parks (not so good), monorails and moving pavements like airport conveyor belts, water copters and underwater buses(after you on the trial run). Not surprisingly after this summer of discontent, the underground doesn't get many votes.

Cycling collects more votes which will be music to Prescott's ears as Britain has the worst record for number of cyclists and the most number of children driven to school by their parents. Riverboats didn't collect many votes but, since they are big pollutants running on diesel that only operate from a few pontoon stations, voting for them as a commuter is the equivalent of spoiling your vote. Riverboat commuter systems are as likely to occur as ferries in the bottom of the garden.

As Horden observes, "a monorail deals realistically with the problem that the tidal Thames is a mud flat with a deep central fast-flowing channel for six to nine hours at a time. To ferry the Thames and have passenger boats plying the river, it has to be dammed. If the Thames is left as a tidal river, we can't use it as an efficient transport system."

The most sexy designs in the exhibition are the alternative forms of transport. This has the unfortunate spin-off that they collect the most votes despite captions pointing out that they are either only prototypical one-offs or prohibitively expensive. Dream on, Mondeo man and Clio woman for a Honda Dream car powered with solar panels.

`Fast Forward>>Rewind<<' is at the London Transport Museum, London WC2 (0171-379 6344), until April 2000