line reopened using newly built Standard Stock electrical multiple units. When the Central London Railway opened in 1900, carriages were hauled by electric locomotives. However, the 44-ton locomotives had a high unsprung weight of 34 tons, and these caused noise and vibrations that could be felt at ground level. In 1902–03 the carriages were reformed into multiple units using a control system developed by Frank Sprague in Chicago. The District Railway used electric locomotives on electrified underground lines that were exchanged for steam locomotives to continue over unelectrified track. Ten were built in 1905, and these operated in pairs, initially used to haul London and North Western Railway passenger trains on their Outer Circle route between Earl's Court and Mansion House. From 1910 trains from the London, Tilbury and Southend Railway were extended over the District, the steam locomotives being exchanged for electric ones at Barking. These locomotives were scrapped after the service was withdrawn in 1939. Similarly Metropolitan Railway electric locomotives were used with conventional carriage stock. On the outer suburban routes an electric locomotive was used at the Baker Street end that was exchanged for a steam locomotive en route. The first ten had a central cab and were known as camel-backs, and these entered service in 1906. A year later another ten units with a box design and a driving position at both ends arrived. These were replaced by more powerful units in the early 1920s. The locomotives were withdrawn from passenger service after electrification to Amersham was completed in 1961, although three were kept as shunters. Carriages The Metropolitan Railway opened in 1863 with gas-lit wooden carriages hauled by steam locomotives. Lighting was provided by gas — two jets in first class compartments and one in second and third class compartments. Initially the carriages were braked with wooden blocks operated from the guards' compartments at the front and back of the train, giving off a distinctive smell. The Metropolitan and District railways both used loco-hauled carriages exclusively until they electrified in the early 20th century. The District railway replaced all its carriages with electric multiple units. The Metropolitan still used loco-hauled carriages on the outer suburban routes, where an electric locomotive at the Baker Street end was exchanged for a steam locomotive en route. Carriages were introduced in the later years of the 19th century that gave a better ride quality, steam heating, automatic vacuum brakes,