Rail automation is the ability to control the movement of a train without a driver and GoA2 is the first Grade of Automation level that accomplishes this by introducing the core rail automation functionality. A GoA2 vehicle controller has full control over the trains braking and propulsion and the decision of when to apply either one.
As a result, the jump from GoA1 to GoA2 is an order of magnitude higher than the jump to any other grade (for example from GoA2 to 3 or GoA3 to 4) due to the complexity and amount of automation required.
In a GoA1 system:
The driver observes the wayside signals or cab signals
The driver controls the propulsion and brakes
The driver performs the complicated function of correlating the wayside signal aspect into train commands to either apply brakes or accelerate
The driver is the critical link between the aspect displayed on a signal and the commands sent to the trains braking and propulsion. Whereas in GoA2, these duties are eliminated and replaced by the vehicle and wayside controller.
The wayside controller determines how far the train is permitted to travel by transmitting movement authorization data to the vehicle controller. The vehicle controller determines when to apply braking or propulsion effort based on the movement authority.
This is the minimum of what is required for signalling automation but also the most complicated. Once a GoA2 design is established and proven, the jump to GoA3 and GoA4 is incremental because the signalling automation architecture is established.
To understand the jump from GoA1 to GoA2 and the GoA2 functionality, the following video dives into greater details.