Judges in a competition launched during the 2003 heat wave to design a cooling system for London's underground have been unable to find a winner.
Temperatures on Tube trains can reach 30C
Of the 3,500 entries, not one was original and workable, said London Underground (LU) director Tim O'Toole.
He said it demonstrates cooling the Tube is "a very complex issue".
Despite the results, LU launched its annual Beat the Heat campaign and said tens of millions of pounds would be invested to address Tube temperatures.
The winner of the competition launched by London mayor Ken Livingstone would have received £100,000 in prize money.
LU has long grappled with the problem of how to reduce heat in the narrow tunnels - some built more than a century ago - which lie up to 60 metres below ground.
Temperatures can top 30C (90F) in the deep tunnels of the network, according to a 2002 survey conducted by Liberal Democrat MP Tom Brake.
Among the ideas were handing out ice-lollies, turning the Tube into a Venetian-style oasis, and putting up posters of snowmen to make people think of winter.
Mr O'Toole said: "We were amazed at the number of entries to the Cooling the Tube competition.
"Unfortunately, no one has come up with a practical, workable solution beyond anything we have already examined or were working on."
The programme for this summer includes a poster campaign reminding passengers to carry a bottle of water, stay off the Tube if unwell and avoid pulling the passenger alarm between stations.
LU said they will introduce the first air-cooled trains on the Circle, District, Hammersmith and City and Metropolitan Lines in 2009.
More than 150 of London's buses will benefit from new ventilation and 1,000 buses will be given more opening windows on the top deck.