London tubes can get overheated during the summer months.
Commuters will be happy to know that London Underground has started testing the first air conditioned tube train.
Heat control has been an ongoing problem on London's tubes, especially in the summer months when passengers have to endure scorching temperatures.
The heat experienced on the underground is created by the energy consumed by the trains and the millions of passengers using the network each day.
The tube is the oldest metro system in the world.
Its rudimentary infrastructure has hardly changed since it was assembled over 140 years ago.
The majority of the tube network was built long before air-conditioning was invented and consequently no provision was made for its installation.
Part of the problem in installing air-conditioning on trains would be the size of the unit that would be required for each carriage.
The first section of the tube to open was between Farringdon and Paddington in 1863
There are 250 underground stations in London
3.5 million people use the tube daily
Underground sections of the sub-surface (Metropolitan Line) lines are rather close to the surface where heat can escape from the system much more readily than from the deep-level tubes (Northern Line).
The new trains scheduled to be unveiled in September 2010 will be longer and larger than current trains and will increase capacity by nearly 20 per cent.
The air conditioned trains will be introduced on the Metropolitan line, followed by the Circle, Hammersmith & City and District Lines.
London Underground is also trying to develop ways to install air conditioning within the network's deep lines, such as the Northern and Victoria line.
When Transport for London took control of the Tube in July 2003, attempts to try to cool the network were not backed by investment, it was claimed.
Transport for London said: "Setting up a dedicated cool the tube team was part TfL's commitment to co-ordinate a programme to understand the problem and to invest to try to tackle heat on the Underground in future."
The dedicated cool the tube team was set up in June 2005.
In 2003, London's mayor, Ken Livingstone offered a prize of £100,000 to the person who found a solution to London's overheated underground.