The museum houses the world’s largest collection of Cornish beam engines, including the largest working beam engine, which has a cylinder diameter of 90 inches and was used to pump water to London for 98 years. There are also several other large working Cornish beam engines, a triple-expansion engine and several rotative steam engines.
In 2008, the museum completed the restoration of its Bull Engine, which is one of only four known examples in the world, and the only engine in its original location. The Bull engine was built in 1856 and first steamed in restoration in 2006.
One of the museum’s Allen Diesel engines is also on display and operated each weekend.
The steam museum is home to London’s only operating steam railway. The 2 ft (610 mm) gauge narrow gauge railway is run by volunteers of the museum and the 2009 season sees the debut of the museum’s new "Wren" locomotive. The line runs for 400 yards around the Kew Bridge site and passenger trains are operated on Sundays during the summer months and on other special days.
The railway was inspired by similar facilities provided at major waterworks in the United Kingdom, notably the Metropolitan Water Board Railway between Hampton and the Kempton Park waterworks.