I came across this article about what TfL is doing to keep the Tube cooler:
http://www.ingenia.org.uk/ingenia/ar...aspx?index=821

Here's an extract:

"Controlling Heat Levels

With a fully loaded train accelerating away from each central London
platform 30 times each hour at peak periods, energy efficiency alone is not
enough to control temperatures to acceptable levels. On the recently
upgraded Victoria Line, the throughput of fresh air has been doubled, and
additional cooling systems have been installed at two stations, with a
third to follow.

The conventional solution to double air throughput would have been to sink
additional ventilation shafts, but property acquisition costs and
timescales have made this unviable. Instead, London Underground has
installed new fans into the existing shafts. The fans have capacities up to
100 m3/s and the greatest challenge has been to control their operating
sound levels. A lot of the fans are situated alongside residential
accommodation and others are in areas that are otherwise very quiet at
night.

Splitter-plate attenuators, each about the size and weight of a small
delivery van, have been fitted on each side of each side of the new fan. In
several cases, it has been necessary to fit the attenuators within the
narrow shafts descending through the tunnels or within the ‘chimneys’ that
convey the warm air out and above the roofs of adjacent buildings. All of
the new fans and the dampers can now be controlled remotely so that air
flows can be shut off or reversed when necessary.

Air Handling Units

At Victoria, Green Park and Oxford Circus stations, there was no viable way
of achieving the amount of cooling required using fans. A review of
alternatives identified water, ideally from a naturally cool source, as the
medium to transport heat energy out of the system. London Underground
worked in partnership with researchers and engineers at London South Bank
University on concept development. To avoid creating condensation problems
and to limit energy consumption, they decided to supply cool, rather than
very cold, water, which meant that large water/air heat exchangers, known
as Air Handling Units (AHUs) would need to be installed. This would give
the required 200 kW of cooling needed for each tunnel at each station.

Owing to the lack of space in tunnels, the only space available for the
AHUs was above the tracks in the station platforms. The London Underground
design team set out to create units that would require little maintenance,
have low energy consumption, be visually acceptable, and which could be
installed during the normal night-time maintenance shutdown of train
services – a window of around four-and-a-half hours.

A small-scale trial system was installed above a concourse within Victoria
station in 2006, using 50 kW AHUs supplied with cool water from an
underground river that drains to a huge sump below the station. The trial
was successful and secured a Carbon Trust Innovation Award in 2007 – see
Developing cooling units.

By June last year, 24 AHUs had been fitted at Green Park and Oxford Circus,
in time to improve the environment for passengers attending the London 2012
Olympic Games. Each unit provides a stream of air, cooled to around 16°C,
that circulates around passengers waiting on the platform, and is then
driven into the tunnel by the piston-action of the moving trains.

Cool water to supply the AHUs is sourced differently at each station. At
Green Park station, water is from the chalk aquifer 130 m below the ground.
The two uptake and two re-injection boreholes were created with the
assistance of the Royal Parks in the adjoining park. At Oxford Circus,
water is circulated to rooftop heat exchangers; ambient air provides most
of the cooling, with refrigerant chiller units supplementing the system in
warm weather. The full system at Victoria will be installed as part of the
major upgrade of the station that is underway, and will use water from the
same sump that is used to supply the trial installation at the site."

More in http://www.ingenia.org.uk/ingenia/ar...aspx?index=821