London Underground rolling stock includes the electric multiple-unit trains used on the London Underground. These come in two sizes, smaller deep-level tube trains and larger sub-surface trains of a similar size to those on British main lines, both running on standard gauge tracks. New trains are designed for the maximum number of standing passengers and for speed of access to the cars. The first underground passenger services started in 1863 when the Metropolitan Railway opened using steam locomotives hauling gas-lit wooden carriages, braked from a guards' compartment. In 1890, the City and South London Railway opened the world's first deep-level tube railway, using electric locomotives pulling carriages with small windows, nicknamed "padded cells". Other tube railways opened in the early 20th century using electric multiple units known as 'gate stock', as access to them was via lattice gates at each end of the car. The earlier railways had electrified the underground sections of their lines by 1907. Pneumatic sliding doors were introduced on tube trains in 1919 and sub-surface trains in the late 1930s. Until the early 1960s an electric locomotive was exchanged for a steam locomotive on Metropolitan line services beyond Rickmansworth. The Victoria line opened in the late 1960s using automatic train operation (ATO), and the last trains ran with a guard in 2000. , the Central, Jubilee, and Northern lines also use forms of ATO, the latter two using a system called TBTC (transmission-based train control). The older sub-surface trains were replaced between 2010 and 2017 by new air-conditioned S Stock, and the replacement of the 1972 Stock and the 1973 Stock on the Bakerloo and Piccadilly lines respectively is currently under consideration. They will be replaced by the New Tube for London. Current stock London Underground trains come in two sizes, larger sub-surface trains and smaller deep-tube trains. Since the early 1960s all passenger trains have been electric multiple units (EMUs) with sliding doors. A train last ran with a guard in 2000. All lines use fixed-length trains with between six and eight cars, except for the Waterloo & City line, which uses four cars. New trains, which are designed for maximum number of standing passengers and for speed of access to the cars, have regenerative braking and public address systems. Since 1999, all new stock has had to comply with accessibility regulations that require such things as access and room for wheelchairs, and the size