ATO is a safety enhancement where the rail vehicle automatically performs key operational functions.

Typically, ATO is Semi-Automatic Train Operation (STO) where the train stops automatically when/where required, but the driver is responsible to start the train, operate doors and respond to emergencies.

Driverless Train Operation (DTO) occurs where starting and stopping are all automated and an attendant only operates doors (and drives the train in emergencies).

Unattended Train Operation (UTO) is where all operations such as starting, stopping, door operation and emergency handling are fully automated.

ATO systems are typically used in rail environments such as subways and elevated railways which safety-wise are free of humans around the track. However Rio Tinto is using DTO in mainline operations in outback Western Australia (also relatively free of humans).