Double-decker trains could be introduced on some of the busiest commuter routes in an attempt to ease overcrowding.

The Brighton to London Bridge service has been earmarked for the experiment along with three other lines out of London.

The Department for Transport (DfT) is investigating double-deckers that would be shorter than the two-tier trains operating in Europe.

The soaring demand for rail travel has seen a growing number of services bursting at the seams during rush hour.

Figures released last year showed that 5,833 of the 75,228 people crammed into Southern Trains carriages in the average weekday morning peak period were on trains already officially classed as full.

And 1,040 of the 58,174 people travelling during the evening rush hour were suffering the same inconvenience.

First Capital Connect, which operates from Brighton to Bedford through London, carries 26,995 passengers in the mornings, of which 651 were classified as "excess" in the figures released in November last year.

Officials estimate the network will run out of capacity within 20 years unless drastic measures are adopted.

Expensive One option would be to raise the cost of rush-hour travel to a level where it deters some travellers but that is considered politically and environmentally unacceptable.

Trains could be made longer but that would require an extensive and expensive platform-lengthening programme, unless the rolling stock was overhauled to allow only certain doors to be opened at smaller stations.

Alistair Darling, the former transport secretary, supports double-deckers as a way of providing more capacity.

Shelley Atlas, chairwoman of Brighton Line Commuters, said raising bridges and tunnels would be a "massive job".

The DfT has asked manufacturers if it would be possible to make a shorter version of the double-decker trains as a means of reducing the cost of any potential track work.

A DfT spokesman said: "We are looking at a number of options to help cater for increased rail demand over the coming years. This includes train lengthening and other infrastructure projects.

"In the longer term the development of double-decker trains could be an answer on a small number of routes but given the likely changes required to the rail network, implementation could be difficult and costly."

Several companies are understood to be involved in talks, including Siemens and Bombardier.

First Capital Connect said it could not comment on the idea because its franchise runs out in nine years, before the trains would be introduced.