Automatic train control

Automatic train control 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 and is usually considered to be the safety-critical part of a railway system.
There have been numerous different safety systems referred to as "automatic train control" over time. The first experimental apparatus was installed on the Henley branch line in January 1906 by the Great Western Railway, although it would now be referred to as an automatic warning system because the driver retained full command of braking. The term is especially common in Japan, where ATC is used on all Shinkansen lines, and on some conventional rail and subway lines, as a replacement for ATS.

Africa

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 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. 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 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:

ATC-1: ATC-1 is used on the Tōkaidō and Sanyō Shinkansen since 1964. The system used on the Tōkaido Shinkansen is classified as ATC-1A and ATC-1B on the Sanyō Shinkansen. Originally utilizing trackside speed limits of, it was upgraded to utilize speed limits of with the introduction of new rolling stock on both lines. Variants include ATC-1D and ATC-1W, the latter being used exclusively on the Sanyō Shinkansen. Since 2006, the Tōkaidō Shinkansen's ATC-1A system has been superseded by ATC-NS.
ATC-2: Used on the Tōhoku, Jōetsu and Nagano Shinkansen routes, it utilized trackside speed limits. In recent years, ATC-2 has been superseded by the digital DS-ATC. The Japanese ATC-2 system is not to be confused with the Ansaldo L10000 ATC system used in Sweden and Norway, which is similar to the EBICAB 700 and 900 ATC systems used in some other parts of Europe. The analog ATC signal has been proved to be safe, but it only provides the allowable maximum speed on the block section. The speed signal is composed of a primary frequency band between 750 ~ 1000 Hz and a secondary frequency band around 1200 Hz. The FSK modulated frequencies, and their corresponding allowable speed are shown at the table below.

Primary Secondary	12.0 Hz	16.5 Hz	21.0 Hz	27.0 Hz	32.0 Hz	38.5 Hz
10.0 Hz		240 km/h	Switching over signal	210 km/h		0 km/h
15.0 Hz			Switching over signal	160 km/h		0 km/h
22.0 Hz	110 km/h		Switching over signal			0 km/h
29.0 Hz	70 km/h		Switching over signal			0 km/h
36.0 Hz			Switching over signal		30 km/h	0 km/h
41.5 Hz			Switching over signal		0 km/h	0 km/h

A major weak point of ATC-2 is that the system cannot provide different instructions to trains with different brake performances due to the limit of bandwidth and modulation technique. The trains with better brake performances may be asked to decelerate too early, and hence wastes the better efficiency and high performance of new trains while old trains are not retired yet.

ATC-3 : Actually the first implementation of ATC in Japan, it was first used on Tokyo Metro Hibiya Line in 1961 and later on the Tokyo Metro Tōzai Line. Stands for Wayside-ATC. Both lines converted to New CS-ATC in 2003 and 2007 respectively. WS-ATC is also used on 5 Osaka Metro lines.
ATC-4 : First used on the Tokyo Metro Chiyoda Line in 1971, CS-ATC, is an analogue ATC technology using ground-based control, and, like all ATC systems, used cab signalling. CS-ATC uses trackside speed limits of 0, 25, 40, 55, 75 and 90 km/h. Its use has extended to include the Tokyo Metro Ginza Line, Tokyo Metro Marunouchi Line, and most recently, the Tokyo Metro Yurakucho Line. It is also used on all Nagoya Municipal Subway lines and 3 Osaka Metro lines.
ATC-5: Introduced on the Sōbu Line and the Yokosuka Line from 1972 to 1976, it utilized trackside speed limits of 0, 25, 45, 65, 75 and 90 km/h. ATC-5 was deactivated on both lines in 2004 in favour of ATS-P.
ATC-6: Introduced in 1972, formerly used on the Saikyō Line, Keihin-Tōhoku Line/Negishi Line and Yamanote Line. Some freight trains were fitted with ATC-6 as well. In 2003 and 2006, the Keihin-Tōhoku and Yamanote Lines replaced their ATC-6 systems with D-ATC. Saikyō Line replaced its ATC-6 system to ATACS in 2017.
ATC-9: Used on the Chikuhi Line in Kyushu.
ATC-10 : Developed from ATC-4, ATC-10 can be partially compatible with D-ATC and completely compatible with the older CS-ATC technology. ATC-10 can be seen as a hybrid of analogue and digital technology, although ATC-10 is not recommended for use with D-ATC because of poor performance of the full-service brake during trial tests. It is used on all Tokyo Metro lines, the Tōkyū Den-en-toshi Line, Tōkyū Tōyoko Line and Tsukuba Express.
ATC-L: Used on the Kaikyō Line along with Automatic Train Stop from 1988–2016. Replaced by DS-ATC following opening of the Hokkaido Shinkansen.
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 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:

Use of one-step brake control permits high-density operations because there is no idle running time due to operation delay between brake release at the intermediate speed limit stage.
Trains can run at the optimum speed with no need to start early deceleration because braking patterns can be created for any type of rolling stock based on data from wayside equipment indicating the distance to the next train ahead. This makes mixed operation of express, local, and freight trains on the same track possible at the optimum speed.
There is no need to change the wayside ATC equipment when running faster trains in the future.
Multiple data can be transmitted to the on-broad computers due to its high data transmission rate, which takes advantage of the modern DSP software and hardware technology.

From a bock diagram demonstrating the demodulation system used in TVM430 standard signaling of China national railway, we see the DSP system consists of a band-pass filter, FFT, and searching peak and harmonics in the spectrum. The entire flow starts with sampling the data, adding hamming window, FFT, calculating power spectrum, and searching throuout the spectrum to generate the sequence of transmitted signal.
To date, the following digital ATC systems are used:

D-ATC: Used on non-high speed lines on some East Japan Railway Company lines. Stands for Digital ATC. Its main difference from the older analog ATC technology is the shift from ground-based control to train-based control, allowing braking to reflect each train's ability, and improving comfort and safety. The fact that it can also increase speeds and provide for denser timetables is important for Japan's busy railways. The first D-ATC was enabled on the section of track from Tsurumi Station to Minami-Urawa Station on the Keihin-Tohoku Line on 21 December 2003 following the conversion of the 209 series trains there to support D-ATC. The Yamanote Line was also D-ATC enabled in April 2005, following the replacement of all old 205 series rolling stock to the new, D-ATC enabled E231 series trains. There are plans to D-ATC enable the rest of the Keihin-Tohoku line and the Negishi line, pending conversion of onboard and ground-based systems. The ATC system on the Toei Shinjuku Line in use from 14 May 2005 is very similar to D-ATC. Since 18 March 2006, Digital ATC has also been enabled for Tōkaidō Shinkansen, the original Shinkansen owned by Central Japan Railway Company, replacing the old analog ATC system. D-ATC is used with the THSR 700T built for the Taiwan High Speed Rail, which opened in early January 2007.
DS-ATC: Implemented on Shinkansen lines operated by JR East. Stands for Digital communication & control for Shinkansen-ATC. It is used on the Tōhoku Shinkansen, Hokkaido Shinkansen, Joetsu Shinkansen and the Hokuriku Shinkansen. DS-ATC is proposed to improve the weakness of ATC-2, by sending train messages consisting of the distance to the preceding train, or start point of speed limit, and a block identification number by the through track circuits. The new system for Tohoku, Hokkaido, Joetsu, and Hokkuriku Shinkansen is named DS-ATC. Compared to the old ATC, DS-ATC does not show the speed command directly to the train drivers, the on-board computers evaluate the distance from the preceding train, braking performance, gradient, position, and commands given by the dispatch center.

Two carrier frequencies at 575 Hz and 625 Hz, and MSK modulation are selected for the signal transmission of DS-ATC. For each block section of the tracks, a TDAT is installed to detect whether there is a train in the block and transmit modulated signals to the train. Transmitters are placed at the end of the block towards the direction of the train, and a set of receivers are placed at the other end. The telegram transmitted are modulated by MSK since the MSK signaling is robust for interference and requires little part of the bandwidth. In the meantime, the TDAT detects the position of trains by comparing the demodulated transmitted and received signals at another end of the block section.
As the modulated frequencies are 575±8 Hz and 625±8 Hz, the occupied bandwidth for both primary and secondary frequency channels are 16 Hz, and a maximum data transmission rate of 64bit/s is achieved. The TDAT feeds the signal in the track circuits, and the trains receive the signals by a pair of pick-up coils that generate induction current from the magnetic field of the tracks. There are five types of telegrams transmitted to the trains. Their items and length of each sequence are listed on the table below.

Type	Items	Length
1	Bit sequence to identify the edge of block sections	24
2	ID number of the block section, speed limit, number of unblocked sections, etc.	64
3	Increased number of unblocked sections	34
4	Decreased number of unblocked sections	34
5	Permissible speed, length, and average gradient in the block	44

Once a train calls 5060B, for example, enters a block, the type 1 telegram is received, and the train identifies the moment as the edge of a block. The time delay between the train enters the block and the on-board computer demodulated the type 1 telegram is a Gaussian distribution based on test runs near Niigata in late 1990s. The mean value and the standard deviation are 514ms and 29.2 respectively, and that means the average edge detection error is 39.26m under a maximum operational speed of 275 km/h. When TDAT senses 5060B entering, it starts to transmit type 2 telegram. Before 5060B leaves the block, the TDAT continues to transmit type 2, unless the number of unblocked sections changes.
A challenge of ATC compared to traditional signal lights is the traction current and thunderstorm. The traction current is supplied to the trains from the overhead cables and returns to the electric substations by the tracks. In other words, there exists a large AC current around 1000A flowing in the tracks. For elevated tracks, the lightning strike near the tracks is a source of EMI, too. Those two may cause a large intermodulation distortion in the track circuits.
The digital encoded ATC is easy to examine with a signal inspection car. With DSP chips, the ATC signal transmission can be collected by an inspection car, the demodulated time domain signals is stored and analyzed on the car by comparing the ideal telegrams in the database and the received telegrams.

RS-ATC: Used on the Tōhoku, Hokkaido, Hokuriku and Jōetsu Shinkansen at a fallback level from DS-ATC. RS-ATC is similar in principle to GSM-R in that radio signals are used to control the speed limit on trains, as compared to trackside beacons and/or transponders on other types of ATC.
ATC-NS: First used on the Tōkaidō Shinkansen since 2006, ATC-NS, is a digital ATC system based on DS-ATC. Also used on the Taiwan High Speed Railway and the San'yō Shinkansen.
KS-ATC: Used on the Kyushu Shinkansen since 2004. Stands for Kyushu Shinkansen-ATC.