Automatic Train Control (ATC)
A brief description of the architecture used by ATC (Automatic Train Control) systems.
More articles on this site about signalling are listed in the Signalling Index
Definition of ATC
To the UK reader, the letters ATC refer to "Automatic Train Control", which was the title given to the warning system tried on some UK lines before the general introduction of the AWS (Automatic Warning System) in the 1960s. In the US it also refers to Automatic Train Control but it refers to a more modern concept where the system includes ATP (Automatic Train Protection), ATO (Automatic Train Operation) and ATS (Automatic Train Supervision). It has been adopted around the world to describe the architecture of the automatically operated railway. Of course, it is usually applied only to metros. This page looks at the relationship between the four different automatic train concepts.
As a definition, ATC (Automatic Train Control) refers to the whole system which includes all the other automatic functions and, for some of these functions at least, also includes a degree of manual intervention. ATC therefore, is the package which includes ATP, ATO and ATS.
The ATC Package
There are a number of ways to assemble the parts of an ATC package but a common format used by many systems looks like this:
The diagram shows the basic architecture of a fixed block automatic train control (ATC) system with its three main components - ATP (Automatic Train Protection), ATO (Automatic Train Operation) and ATS (Automatic Train Supervision). The basic safety requirement, to keep trains a safe distance apart, is performed by the ATP, which has a control unit for each block. This control unit receives the data from the blocks ahead, converts that into a speed limit for the block it controls and sends the speed limit data to the track. The train picks up the data using the codes transmitted along the track. The transmission system can be track circuits, loops or beacons (balises) located along the track. For more details see ATP Code Transmission.
The data received by the ATP control unit is usually limited to indicating that a train is in the block or the speed limit currently imposed in the block. This data is sent to the ATS computer where it is compared with the timetable to determine if the train is running according to schedule or is late or early. To adjust the train's timing, the ATS can send commands to the ATO spots located along the track.
The ATO spots, which can be short transmission loops or small boxes called beacons or "balises", give the train its station stop commands. The spots usually contain fixed data but some, usually the last one in a station stop sequence, transmit data about the time the train should stop (the dwell time) at the station and may tell it how fast to go to the next station (ATP permitting).
Some systems leave the ATO spots alone - i.e their data is always fixed - but use the ATP system to prevent the train from starting or restrict its speed. The ATS computer tells the ATP control unit to transmit a restricted speed or zero speed to the track.
Both ATP and ATO commands are picked up by aerials on the train and translated into motoring, braking or coasting commands. Where a train can be manually driven, the ATP will still ensure the safety requirement but the ATO is overridden, the driver stopping the train in the stations by use of the cab controls.
There are lots of variations of ATC around the world but all contain the basic principle that ATP provides safety and is the basis upon which the train is allowed to run. ATO provides controls to replace the driver, while ATS checks the running times and adjusts train running accordingly.
There is little difference between fixed block and moving block as far as ATC is concerned but the architecture will look something like this:
The transmission of data to the rails is gone and is replaced by radio transmission. Also, there are no blocks. The train's location is determined by the on-board route map, which is reset when the train starts its trip and is verified by "checking balises" spaced along the route. The balises can be used to send ATS instructions to the train but, like the ATO spots used in fixed block systems, they contain static data about location and route profile.
In a moving block system, the ATP control unit differs from that used in a fixed block system. It now covers a larger area and it gets its data from the radio transmissions. It sends data by radio as well. If the radio transmission fails to reach a train, this train assumes that the train in front has stopped at its last known position and will stop a safe distance behind it.
ATS covers the same functions as for fixed block systems. Train location data is received and train running adjusted as necessary. In all ATS systems nowadays, there is lots of data logging to provide management information and statistics and some ATS systems allow replays of sections of the day's train movements to assist in formulating future recovery management strategies.
There are some variations on the general principles mentioned above and these are noted in the article on Moving Block signalling.
More articles on signalling:
The Development and Principles of UK Signalling - Metro Signalling and ATP - ATP Transmission and Moving Block Systems - ATO - Route Signalling - UK Warning & Protection Systems - UK Signalling - What the driver sees - Single Line Signalling - US Signalling - Docklands Light Railway Signalling