Crew size and automation

In the early days of underground railways, at least two staff members were needed to operated each train: one or more attendants (also called "conductor" or "guard") to operate the doors or gates, as well as a driver (also called the "engineer" or "motorman"). The introduction of powered doors around 1920 permitted crew sizes to be reduced, and trains in many cities are now operated by a single person. Where the operator would not be able to see the whole side of the train to tell whether the doors can be safely closed, mirrors or closed-circuit TV monitors are often provided for that purpose.

A replacement system for human drivers became available in the 1960s, with the advancement of computerized technologies for automatic train control and, later, automatic train operation (ATO). ATO could start a train, accelerate to the correct speed, and stop automatically in the correct position at the railway platform at the next station, while taking into account the information that a human driver would obtain from lineside or cab signals. The first metro line to use this technology in its entirety was London's Victoria line, opened in 1968. In normal operation, a crew member sits in the driver's position at the front, but is only responsible for closing the doors at each station. By pressing two "start" buttons the train would then move automatically to the next station. This style of "semi-automatic train operation" (STO), known technically as "Grade of Automation (GoA) 2", has become widespread, especially on newly built lines like the BART network in the San Francisco Bay Area.

A variant of ATO, "driverless train operation" (DTO) or technically "GoA 3", is seen on some systems, as in London's Docklands Light Railway, which opened in 1987. Here, a "passenger service agent" (formerly called "train captain") would ride with the passengers rather than sit at the front as a driver would, but would have the same responsibilities as a driver in a GoA 2 system. This technology could allow trains to operate completely automatically with no crew, just as most elevators do. When the initially increasing costs for automation began to decrease, this became a financially attractive option for employers. At the same time, countervailing arguments stated that in an emergency situation, a crew member on board the train would have possibly been able to prevent the emergency in the first place, drive a partially failed train to the next station, assist with an evacuation if needed, or call for the correct emergency services and help direct them to the location where the emergency occurred. In some cities, the same reasons are used to justify a crew of two rather than one; one person drives from the front of the train, while the other operates the doors from a position farther back, and is more conveniently able to assist passengers in the rear cars. An example of the presence of a driver purely due to union opposition is the Scarborough RT line in Toronto.

Completely unmanned trains, or "unattended train operation" (UTO) or technically "GoA 4", are more accepted on newer systems where there are no existing crews to be displaced, and especially on light metro lines. One of the first such systems was the VAL (véhicule automatique léger or "automated light vehicle"), first used in 1983 on the Lille Metro in France. Additional VAL lines have been built in other cities such as Toulouse, France, and Turin, Italy. Another system that uses unmanned trains is Bombardier's Innovia Metro, originally developed by the Urban Transportation Development Corporation as the Intermediate Capacity Transit System (ICTS). It was later used on the SkyTrain in Vancouver, British Columbia, which carries no crew members, and the Kelana Jaya Line in Kuala Lumpur, Malaysia.

Systems which use automatic trains also commonly employ full-height platform screen doors or half-height automatic platform gates in order to improve safety and ensure passenger confidence, but this is not universal, as networks like Nuremberg do not, using infrared sensors instead to detect obstacles on the track. Conversely, some lines which retain drivers or manual train operation nevertheless use PSDs, notably London's Jubilee Line Extension. The first network to install PSDs on an already operational system was Hong Kong's MTR, followed by the Singapore MRT.

As for larger trains, the Paris Métro has human drivers on most lines but runs automated trains on its newest line, Line 14, which opened in 1998. The older Line 1 was subsequently converted to unattended operation by 2012, and it is expected that Line 4 will follow by 2019. The North East MRT Line in Singapore, which opened in 2003, is the world's first fully automated underground urban heavy-rail line. The MTR Disneyland Resort Line is also automated, along with trains on the South Island Line.