Automatic train control
Automatic train control (ATC) is a general class of train protection systems for railways that involves a speed control mechanism in response to external inputs. For example, a system could effect an emergency brake application if the driver does not react to a signal at danger. ATC systems tend to integrate various cab signalling technologies and they use more granular deceleration patterns in lieu of the rigid stops encountered with the older automatic train stop technology. ATC can also be used with automatic train operation (ATO) and is usually considered to be the safety-critical part of  the system. Over time, there have been many different safety systems labeled as "automatic train control". The first was used from 1906 by the Great Western Railway, although it would now be referred to as an AWS (automatic warning system). The term is especially common in Japan, where ATC is used on all Shinkansen (bullet train) lines, and on some conventional rail lines, as a replacement for ATS. Africa. Egypt. The accident report for the 2006 Qalyoub accident mentions an ATC system. South Africa. In 2017, Huawei was contracted to install GSM-R partly to provide communication services to automatic  train protection systems. Asia. Japan. In Japan, the Automatic Train Control (ATC) system was developed for high-speed trains like the Shinkansen, which travel so fast that the driver has almost no time to acknowledge trackside signals. Although the ATC system sends AF signals carrying information about the speed limit for the specific track section along the track circuit. When these signals are received on board, the train's current speed is compared with the speed limit and the brakes are applied automatically if the train is travelling too fast. The brakes are released as soon as the train slows  below the speed limit. This system offers a higher degree of safety, preventing collisions that might be caused by driver error, so it has also been installed in heavily used lines, such as Tokyo's Yamanote Line and some subway lines. Although the ATC applies the brakes automatically when the train speed exceeds the speed limit, it cannot control the motor power or train stop position when pulling into stations. However, the automatic train operation (ATO) system can automatically control departure from stations, the speed between stations, and the stop position in stations. It has been installed in some  subways. However, ATC has three disadvantages. First, the headway cannot be increased due to the idle running time between releasing the brakes at one speed limit and applying the brakes at the next slower speed limit. Second, the brakes are applied when the train achieves maximum speed, meaning reduced ride comfort. Third, if the operator wants to run faster trains on the line, all the related relevant wayside and on-board equipment must be changed first. Analogue ATC. The following analogue systems have been used: Digital ATC. The digital ATC system uses the track circuits to detect the presence  of a train in the section and then transmits digital data from wayside equipment to the train on the track circuit numbers, the number of clear sections (track circuits) to the next train ahead, and the platform that the train will arrive at. The received data is compared with data about track circuit numbers saved in the train on-board memory and the distance to the next train ahead is computed. The on-board memory also saves data on track gradients, and speed limits over curves and points. All this data forms the basis for ATC decisions when controlling the  service brakes and stopping the train. In a digital ATC system, the running pattern creates determines the braking curve to stop the train before it enters the next track section ahead occupied by another train. An alarm sounds when the train approaches the braking pattern and the brakes are applied when the braking pattern is exceeded. The brakes are applied lightly first to ensure better ride comfort, and then more strongly until the optimum deceleration is attained. The brakes are applied more lightly when the train speed drops to a set speed below the speed limit. Regulating the braking force  in this way permits the train to decelerate in accordance with the braking pattern, while ensuring ride comfort. There is also an emergency braking pattern outside the normal braking pattern and the ATC system applies the emergency brakes if the train speed exceeds this emergency braking pattern. The digital ATC system has a number of advantages: To date, the following digital ATC systems are used: South Korea. Several subway lines in South Korea use ATC, in some cases enhanced with ATO. Busan. All lines use ATC. All lines are enhanced with ATO. Seoul. Other than on Lines 1 and 2  (MELCO cars only), all lines use ATC. Line 2 (VVVF cars), Line 5 cars, Line 6 cars, Line 7 cars, and Line 8 cars have their ATC systems enhanced with ATO. Europe. Denmark. Denmark's system of ATC (officially designated ZUB 123) is different from that of its neighbours. From 1978 until 1987, the Swedish ATC system was trialled in Denmark, and a new Siemens-designed ATC system was implemented between 1986 and 1988. In consequence of the Sorø railway accident, which occurred in April 1988, the new system was progressively installed on all Danish main lines from the early  1990s onwards. Some trains (such as those employed on the Øresundståg service and some X 2000 trains) have both the Danish and the Swedish systems, while others (e.g. ten of the ICE-TD trains) are fitted with both the Danish and the German systems. The ZUB 123 system is now considered by Banedanmark, the Danish railway infrastructure company, to be obsolete and the entire Danish rail network is expected to be converted to ETCS Level 2 by 2030. The ZUB 123 system is however not used on the Copenhagen S-train network, where another, incompatible safety system called HKT has  been in use since 1975, as well as on the Hornbæk Line, which uses a much more simplified ATP system introduced in 2000. This system is gradually being replaced by the modern and worldwide CBTC signalling standard as of 2022. Norway. Bane NOR—the Norwegian government's agency for railway infrastructure—uses the Swedish system of ATC. Trains can therefore generally cross the border without being specially modified. However, unlike in Sweden, the ATC system used in Norway differentiates between partial ATC ("delvis ATC", DATC), which ensures that a train stops whenever a red signal is passed, and full ATC  (FATC), which, in addition to preventing overshooting red signals, also ensures that a train does not exceed its maximum allowed speed limit. A railway line in Norway can have either DATC or FATC installed, but not both at the same time. ATC was first trialled in Norway in 1979, after the Tretten train disaster, caused by a signal passed at danger (SPAD), occurred four years earlier. DATC was first implemented on the section Oslo S - Dombås - Trondheim - Grong between 1983 and 1994, and FATC was first implemented on the Ofoten Line in 1993. The high-speed Gardermoen Line has had  FATC since its opening in 1998. After the Åsta accident occurred in 2000, the implementation of DATC on the Røros Line was accelerated, and it became operational in 2001. Sweden. In Sweden the development of ATC started in the 1960s (ATC-1), and was formally introduced in the early-1980s together with high-speed trains (ATC-2/Ansaldo L10000). As of 2008, 9,831 km out of the 11,904 km of track maintained by Swedish Transport Administration—the Swedish agency responsible for railway infrastructure—had ATC-2 installed. However, since ATC-2 is generally incompatible with ERTMS/ETCS (as in the case of the Bothnia Line  which is the first railway line in Sweden to exclusively use ERTMS/ETCS), and with the aim of Trafikverket to eventually replace ATC-2 with ERTMS/ETCS over the next few decades, a Special Transmission Module (STM) has been developed to automatically switch between ATC-2 and ERTMS/ETCS. United Kingdom. In 1906, the Great Western Railway in the UK developed a system known as "automatic train control". In modern terminology, GWR ATC is classified as an automatic warning system (AWS). This was an intermittent train protection system that relied on an electrically energised (or unenergised) rail between, and higher than, the  running rails. This rail sloped at each end and was known as an ATC ramp and would make contact with a shoe on the underside of the passing locomotive. The ramps were provided at distant signals. A development of the design, intended for use at stop signals, was never implemented. If the signal associated with the ramp was at caution, the ramp would not be energised. The ramp would lift the shoe on the passing locomotive and start a timer sequence at the same time sounding a horn on the footplate. If the driver failed to acknowledge this warning within  a preset time, the brakes of the train would be applied. In testing, the GWR demonstrated the effectiveness of this system by sending an express train at full speed past a distant signal at Caution. The train was brought safely to a stand before reaching the home signal. If the signal associated with the ramp was clear, the ramp was energised. The energized ramp would lift the shoe on the passing locomotive and cause a bell to sound on the footplate. If the system were to fail then the shoe would remain unenergised, the caution state; it therefore failed safe , a fundamental requirement of all safety equipment. The system had been implemented on all GWR main lines, including Paddington to Reading, by 1908. The system remained in use until the 1970s, when it was superseded by the British Rail Automatic Warning System (AWS). North America. United States. ATC systems in the United States are almost always integrated with existing continuous cab signalling systems. The ATC comes from electronics in the locomotive that implement some form of speed control based on the inputs of the cab signalling system. If the train speed exceeds the maximum speed allowed for that portion of  track, an overspeed alarm sounds in the cab. If the engineer fails to reduce speed and/or make a brake application to reduce speed a penalty brake application is made automatically. Due to the more sensitive handling and control issues with North American freight trains, ATC is almost exclusively applied to passenger locomotives in both inter-city and commuter service with freight trains making use of cab signals without speed control. Some high-volume passenger railroads such as Amtrak, Metro North and the Long Island Rail Road require the use of speed control on freight trains that run on all or  part of their systems. While cab signalling and speed control technology has existed since the 1920s, adoption of ATC only became an issue after a number of serious accidents several decades later. The Long Island Rail Road implemented its Automatic Speed Control system within its cab signalled territory in the 1950s after a pair of deadly accidents caused by ignored signals. After the Newark Bay Lift Bridge Disaster the state of New Jersey legislated use of speed control on all major passenger train operators within the State. While speed control is currently used on many passenger lines in the United  States, in most cases it has been adopted voluntarily by the railroads that own the lines. Currently only three freight railroads, Union Pacific, Florida East Coast and CSX Transportation, have adopted any form of ATC on their own networks. The systems on both FEC and CSX work in conjunction with pulse code cab signals, which in the case of CSX was inherited from the Richmond, Fredericksburg and Potomac railroad on its single main line. Union Pacific's was inherited on portions of the Chicago and Northwestern east–west main line and works in conjunction with an early two aspect cab  signaling system designed for use with ATC. On CSX and FEC more restrictive cab signal changes require the engineer to initiate a minimum brake application or face a more severe penalty application that will bring the train to a stop. Neither system requires explicit speed control or adherence to a braking curve. The Union Pacific system requires an immediate brake application that cannot be released until the train's speed has been reduced to (for any train traveling above that speed). Then, the train's speed must be further reduced to no more than within 70 seconds of the  initial cab signal drop. Failure to apply the brakes for these speed reductions will result in a penalty application. All three freight ATC systems provide the engineer with a degree of latitude in applying brakes in a safe and proper manner, since improper braking can result in a derailment or a runaway. None of the systems are in effect in difficult or mountainous terrain. Further reading. The Railway Technical Website: Automatic Train Control