The London Underground, one of the world's most iconic subway systems, got its first air-conditioned train last week, more than 40 years after its New York rival.
London's subway system - known affectionately by both locals and visitors as "the Tube" - launched its first air-conditioned train on the Metropolitan line on August 4, albeit on a stretch of track which is almost entirely open-air.
Air conditioning is seen as the holy grail for London's sprawling subway network, which becomes unbearably hot during the summer months thanks to the deep lines and lack of ventilation - during 2006, temperatures inside the trains reached as high as 47 degrees Celsius.
However, because of the cramped tunnels and the age of some of the infrastructure (London's subway is the oldest in the world), there simply isn't enough space to install units on trains or to displace the heat that would be extracted from them, which has resulted in London's system facing some very unfavourable comparisons with New York's Subway system or the brilliantly-cooled networks in Asian cities such as Hong Kong.
The new trains rolled out last week will eventually cover some 40 per cent of the network, says Transport for London, but for the deep-running Northern, Bakerloo and Piccadilly line trains, there is still no solution in sight.
"Air-conditioning for the deep-lying lines is a long-term project," a Transport for London spokeswoman told the UK Press Association.
"We are improving conditions on Victoria line trains but there is a lack of space for air-conditioning units on these deep-lying lines. We are looking at various solutions but you have to remember we are dealing with infrastructure which is very old."
In Paris, the world's second oldest system, Metro trains run "enhanced air circulation" systems to cool the carriages, although few tunnels run as deep as those in London, mitigating the problem somewhat.
New York's Subway system introduced its first air-conditioned carriages in 1967, after spending two decades trying to produce a system small enough to fit into the trains, and now the entire fleet of 5800 trains is air-conditioned.
In Moscow, where underground temperatures have soared in recent weeks as the result of a prolonged heatwave, a consumer rights group has filed a lawsuit against city authorities over the temperature, which it says exceeds "sanitary standards".
The head of the Moscow subway's press service said the Metro was introducing trains with air conditioning and working to improve ventilation.
The World's Most Impressive Subways
As selected by Wired Magazine
1. Tokyo Metro
2. Moscow Metro
3. New York Subway
4. London Underground
5. Berlin U Bahn
6. Paris Metro
7. Shanghai Metro
8. Hong Kong MTR
9. Metro Bilbao
10. Chicago L