Double-decker trains could be introduced on British railways in a bid to ease overcrowding at peak times.
Designers are said to be working on ways of developing such trains that would still be compatible with low bridges and tunnels, according to The Times.
Such carriages would be lowered towards the tracks to create height, while aisles on both decks, positioned down the side of the compartment, would create maximum space.
Double decker trains are already used in other parts of Europe.
The idea has been mooted several times in the UK but subjected to long delays due to the need for extensive work required, including raising bridges and building new tunnels.
The Rail Safety and Standards Board (RSSB), which is running the design competition, said the new concepts could be introduced on the country's busiest lines between 2020 and 2022 to "address the challenges facing the UK rail industry".
Photo: Andreas Vogler
The project, led by the Swiss architect Andreas Vogler, is among three at an advanced stage as part of a £3.5 million project to improve conditions on British railways. Another involves a passenger train that can carry freight at off-peak periods.
A demonstration model of the new design — dubbed the Aeroliner 3000 — will be built next year and presented at trade shows.
Trevor Bradbury, from the RSSB, said: "When you walk on to a train nowadays, you step up on to the carriage. There is actually a lot of space that you can use beneath the vehicle, which is what this design does."
The trains were last used on the Dartford to Cannon Street line between 1949 and 1971, but were ditched because passengers found them cramped and uncomfortable.
Recent figures suggested that almost 250,000 commuters were forced to stand on trains each morning, with services running at 3.5 per cent overcapacity.
Photo: Andreas Vogler
The 4.22am service from Glasgow to Manchester airport, which is run by TransPennine Express, is officially the busiest in the country, running at 186 per cent of capacity at its peak.
In September, Claire Perry, the rail minister, said more was being done to ease the strain on the rail network. She pledged more than 3,700 extra carriages across the UK by the end of 2019.