Electric trains were first used in subways in 1890 (on the City and South London line in Britain), and all subsequent underground railways have been electrically worked. Subsurface cut-and-cover lines ‘everywhere are also electrically worked. The early locomotives used on underground railways have given way to multiple-unit trains, with separate motors at various points along the train driving the wheels, but controlled from a single driving cab. With most subway systems, steel wheels running on steel track are used, but some systems have been developed using pneumatic tires running on tracks. These are claimed to offer a smoother ride and give greater traction that allows more rapid acceleration and braking.
Modern subway rolling stock usually has plenty of standing space to hold peak-hour crowds, and a large number of doors, usually opened and closed by the driver or guard, so that passengers can enter and leave the trains quickly at the many, closely spaced stations. Average underground railway speeds are not high – often between 20 and 25 mph (32 to 40 km/h) including stops – but the trains are usually much quicker, as well as cheaper and safer, than surface transportation in the same area.
Where trains emerge into the open on the edge of cities, and stations are a greater distance apart, they can often attain well over 60 mph (97 km/h).
The track and electricity supply are usually much the same as that of main-line railways and most subway lines use forms of automatic signaling worked by the trains themselves and similar to that used by orthodox railway systems. The track circuit is the basic component of automatic signaling of this type on all kinds of railways. Underground railways rely heavily on automatic signaling because of the close operational headways, the short time intervals between trains.
Some railways have no signals in sight, but the signal aspects – green, yellow and red – are displayed to the driver in the cab of the train. Great advances are also being made with automatic driving, now in use in a number of cities. The Victoria Line system in London, England, for long the most comprehensive if not the first, uses codes in the rails for both safety signaling and automatic driving purposes, the codes being picked up by coils on the trains and passed to the driving and monitoring equipment. The high level of control on this line would allow it to operate without drivers, though this practice has not yet been adopted.
Code systems are used on other subways and sometimes they feed information to a central computer which calculates where the train should be at any given time and instructs the train to slow down, speed up, stop, or take any other action needed.