The step, which would require substantial modifications to bridges and tunnels, is one of a number of options being considered to cope with expected passenger and freight growth of 30 per cent over the next 20 years.

Alistair Darling, transport secretary, told a London conference organised by Rail magazine he would examine rigorously the possibility of building a dedicated, high-speed line north from London. He went on to outline his department?s thinking on the project to replace hundreds of ageing Inter-City 125 trains that operate long- distance services on many unelectrified routes.

The three projects will be examined in greater detail next year, when Mr Darling promised to publish a strategy for the next 20-30 years. Passenger rail traffic has grown by 40 per cent in the 10 years since privatisation.

Double-decker trains would be appearing for the first time since 1971, when British Rail withdrew the last of two experimental trains designed and built in 1949 by Oliver Bulleid, a visionary railway engineer. The trains were intended to address the same problem facing modern railway managers: crowded trains calling at short platforms.

They are used commonly in continental Europe and North America, where rail networks have more generous structure gauges ? the technical term for the amount of space allowed for trains through tunnels and under bridges. There was no enhancement to the structure gauge to accommodate the Bulleid trains but they were unacceptably cramped as a result.

The department is likely to explore alternatives such as longer trains or longer platforms before agreeing to work to enhance some lines? structure gauges to accommodate taller trains.

However, for some lines, work to raise bridges and deepen tunnels could prove less complicated than that required to introduce longer trains, which can interfere with signalling equipment and block junctions near stations.

After his speech, Mr Darling said that many of the lines built by the Victorians in the 19th century had two tracks rather than four, which would have allowed for greater capacity.

?There may be cases where we can expand capacity by introducing double-decker trains,? he said. ?Obviously there?s expenditure in dealing with the gauge problem. That may be a better alternative than trying to acquire land and build railways in areas that are already built-up.?

Mr Darling said double-decker trains could be useful around several big conurbations. However, experts believe the idea will be viable mainly on commuter routes south of London.

Iain Coucher, deputy chief executive of Network Rail, the rail infrastructure owner, said the problem was most acute where there were many bridges and tunnels and where there were overhead wires, which would need to be raised.

But double-decker trains could be a good idea elsewhere. ?Where it?s flat, if there are not too many tunnels and bridges, it becomes an attractive option compared with extending platforms,? he said.

Some of the areas with the worst crowding, including South West Trains routes from south-west London, Surrey and Hampshire into London Waterloo, have favourable geography and are electrified with a third rail that could remain.

Work to improve clearances above the track on such routes might cost only a few tens of millions, to judge by similar work done to accommodate taller shipping containers.

Lord Berkeley, head of the Railfreight Group, told the conference routes cleared for double-decker passenger trains could also improve access for trains carrying the taller containers.

On the replacement of the Inter-City 125 trains, Mr Darling said he would be looking to use more energy- efficient technologies in the vehicles, likely to cost a total £1bn. The DfT is co-ordinating the order to achieve economies of scale in buying the trains, to be used by several operators. Mr Darling added a high-speed line could address some transport problems but the issue was complex.

See also:

Good design key to getting rolling stock on track

Financial Times: March 15 2006
By Robert Wright

The key to the future of the busiest railways could be two ageing carriages lying in railway sidings: the last survivors of British Railways? Southern Region?s 4DD stock, the only double-deck trains to operate in Britain, writes Robert Wright.

The trains were a typically daring response by Oliver Bulleid, chief mechanical engineer of the Southern Region, to the challenge of accommodating more passengers on trains between London and Dartford, where platforms were hard to extend.

Bulleid was often an eccentric innovator, once designing a revolutionary steam locomotive resembling a modern diesel, with cabs at both ends.

Because of the extra space needed for steps up to the upper compartments and space constraints, each four-car 4DD train carried only an extra 122 seats above the 386 on a normal four-car train of the time. Modern double-deckers have 40 per cent more seats than single- deck trains.

The Bulleid trains suffered because passengers unfolding themselves out of the cramped compartments took far longer to get off the trains than with normal rolling stock, lengthening times in stations.

The experimental trains were eventually restricted to off-peak services ? at times when their extra capacity was needed least. However, the popularity of modern double-deck trains in France, the Netherlands, Germany, Switzerland and other parts of the world suggests good designs need not suffer such an ignoble end.