Automatic train operation (ATO) is an operational safety enhancement device used to help automate operations of trains. This is achieved according to the Grade of Automation (GoA) present, up to GoA 4 level, where the train is automatically controlled without the presence of staff on board. Mainly, it is used on automated guideway transits and rapid transit systems which are easier to ensure safety of humans. Most systems elect to maintain a driver (train operator) to mitigate risks associated with failures or emergencies.
Many modern systems are linked with Automatic Train Control (ATC) and in many cases Automatic Train Protection (ATP) where normal signaller operations such as route setting and train regulation are carried out by the system. The ATO and ATC/ATP systems will work together to maintain a train within a defined tolerance of its timetable. The combined system will marginally adjust operating parameters such as the ratio of power to coast when moving and station dwell time, in order to bring a train back to the timetable slot defined for it.
Types of train automation
- On the London Underground, the Victoria, Central, Northern, and Jubilee lines run with ATO. The Victoria line, opened in 1968, was the world's first full scale automatic railway and also the first to have an ATO system replaced. The Circle, District, Hammersmith & City and Metropolitan lines are currently being modernised with a brand new automatic train control system.
- The Glasgow Subway has been using ATO since 1980.
- The PATCO Speedline between Philadelphia, Pennsylvania and Lindenwold, New Jersey, opened its first segment in 1969 as the first ATO line in the United States. (The Expo Express, which ran during the World's Fair Expo 67 in Montreal, was the first in North America.)
- On the MTR Network in Hong Kong, all lines operated by MTR Corporation have run with ATO since 1979. The former KCR East Rail Line network has used ATO since 2002.
- Many trains in Japan use some level of ATO.
- Bay Area Rapid Transit (BART), opened in 1972, was the first new metro system with multiple lines built with ATO.
- The Vancouver SkyTrain in Vancouver, British Columbia, is an automated and driverless system commissioned in 1985.
- On the Mass Rapid Transit (Singapore), all lines operating currently run with ATO since 1987.
- All of the lines on the Docklands Light Railway in London have been using ATO (GoA 3) since it opened in 1987.
- On the Nuremberg U-Bahn, existing U2 and new U3 lines converted to ATO, with one-year mix service.
- On the Barcelona Metro, the L9 (as Europe's longest driverless line), L10 and L11 run with ATO.
- The Tren Urbano, which serves the San Juan metropolitan area, has a Siemens ATC system that allows for fully automatic operation.
- On the Milan Metro, the M1 Red Line runs with ATO.
- São Paulo Metrô, Line 4, opened 2010, is the first system operating GoA 4 in South America.
- On the New York City Subway, the BMT Canarsie Line (L train) began full ATO in June 2012. IRT Flushing Line (7 and <7> trains) are undergoing track and signal modernization, with completion in 2018.
- On the Thameslink railway in the core section between St. Pancras and Blackfriars
- The trains on Dubai metro don't have a driver, neither do the trains on AnsaldoBreda Driverless Metro and some trains of the Rome Metro
- The trains on Doha metro recently started operation, running automatically in ATO without a driver 
- Aerotrain (KLIA) in Kuala Lumpur, Malaysia is an automatic train.
- On the Los Angeles Metro system, the Red Line, Purple Line, and Green Line use the GoA 2 ATO system.
- Indonesia's capital city, Jakarta, has a recently operating public transport (Jakarta MRT) that uses the ATO with GoA 2 Level.
- New South Wales's capital city Sydney in Australia, has the Sydney Metro which opened in 2019 and operates with GoA Level 4. Sydney is also in the process of upgrading older lines in the network to eventually enable GoA 2 and higher in the future.
- The Rio Tinto Group "AutoHaul" system on its iron ore railways in the Pilbara. This system is GoA 4 capable, able to run trains without a single person aboard for the entire trip out to the mines and back to the port. In October 2017 the first fully autonomous test took place over a 100 kilometres (62 mi) section. The group was granted accreditation by Australia’s Office of the National Rail Safety Regulator, approving the autonomous operation of iron ore trains in the Pilbara region of Western Australia.
Many railways are planning on using ATO.
- List of automated urban metro subway systems
- Automation of the London Underground
- Communications-based train control – A moving block signalling system that can be used to automate operation of trains
- One-man operation – A method of train operation, sometimes seen as an intermediate step towards greater automation
- Personal Rapid Transit
- Autonomous car
- Guided bus