These are the Kempton Park Steam Engines in London. They are huge. Built between 1926 and 1929, one of the engines, The Sir William Prescott, has been restored to running order and is the largest operational triple expansion steam engine in the world. At 19 meters and 6 stories tall, weighing over 800 tonnes, they are similar to the engines used on the Titanic, and pumped 72 million liters of water a day, supplying north London with drinking water taken from the Thames.
The engines were decommissioned in 1980, and since then the building has been converted to a museum and one of the engines has been restored, while the other is in the process of being restored. It's all being done by a bunch of nice old chaps - steam engine enthusiasts - in their spare time, and it looks like a pretty serious job. It is also used as a set for all sorts of movies and TV shows like Doctor Who - its easy to see why. You can see the engine running a few times each year, to find out when have a look here: http://www.kemptonsteam.org/opening.html
