LGV Signalling

LGV signalling, or in-cab signalling on lignes à grande vitesse (high-speed railway lines based on the French TGV system), differs considerably from signalling on conventional railway lines. Because TGV trains travel too fast for their operators to see and react to traditional lineside signals, an automated system called TVM (Transmission Voie-Machine, or track-to-train transmission) is used for signalling on LGVs. Information is transmitted to trains via electrical pulses sent through the rails. An antenna under the train picks up the signal and an onboard computer decodes the signals, providing speed, target speed, and stop/go indications directly to the operator via dashboard-mounted instruments. This high degree of automation does not remove the train from driver control, though there are safeguards that can safely bring the train to a stop in the event of driver error.

The boundaries of signalling block sections are marked by distinctive boards.
The line is divided into signal blocks of about 1500 m (1 mile), the boundaries of which are marked by blue boards printed with a yellow triangle. Dashboard instruments show the maximum permitted speed for a train's current block, as well as a target speed based on the profile of the line ahead. The maximum permitted speed is based on factors such as the proximity of trains ahead (with steadily decreasing maximum permitted speeds in blocks closer to the rear of the next train), junction placement, speed restrictions, the top speed of the train and distance from the end of LGV track. As trains cannot usually stop within one signal block (which ranges from a few hundred metres to a few kilometres), drivers are alerted to slow down gradually several blocks before a required stop.
Two versons of TVM signalling, TVM-430 and TVM-300, are in use on the LGV. TVM-430, a newer system, was first installed on the LGV Nord to the Channel Tunnel and Belgium, and supplies trains with more information than TVM-300. Among other benefits, TVM-430 allows a train's on-board computer system to generate a continuous speed control curve in the event of an emergency brake activation, effectively forcing the driver to reduce speed safely without releasing the brake.
The signalling system is permissive; the driver of a train is permitted to proceed into an occupied block section without first obtaining authorization. Speed in this situation is limited to 30 km/h (19 mph; proceed with caution) and if speed exceeds 35 km/h (22 mph), the emergency brake is applied and the train stops. If the board marking the entrance to the block section is accompanied by a sign marked NF, the block section is not permissive, and the driver must obtain authorization from the Poste d'Aiguillage et de Régulation (Signalling and Control Centre) before entering. Once a route is set, or the PAR has provided authorization, a white lamp above the board is lit to inform the driver. The driver then acknowledges the authorization using a button on the train's control panel. This disables the emergency braking which would otherwise occur when passing over the ground loop adjacent to the non-permissive board.
When trains enter or leave LGVs from lignes classiques, they pass over a ground loop which automatically switches the driver's dashboard indicators to the appropriate signalling system. For example, a train leaving the LGV onto a French ligne classique would have its TVM signalling system deactivated and its traditional KVB (Contrôle Vitesse par Balise, or beacon speed control) system enabled.
The TVM system was developed by the French group CSEE. It is one of the more advanced railway signaling systems in the world, although this should be kept in perspective as it relies on somewhat antiquated components, such as relays.

How does it work?
There are two components to the TVM-430 system: one ground-based, the other on board the train. Both run using Motorola 68020 class processors, such as those found in early models of the Apple Macintosh, and are programmed in Ada, a computer language often used in safety critical systems. The system makes extensive use of redundancy; the mean time between dangerous failures is estimated to be over 1 million years.
The ground-based segment of TVM-430 resides in trackside boxes, which control stretches of track about 15 km (10 mi) long. Each one is linked to the line's centralized traffic control centre, and directly controls about ten blocks of track, each with its own track circuit. Signaling information is encoded in AC signals which are fed into the rails of each block. There are four different carrier frequencies available in TVM-430, and they are used alternatingly in pairs on both tracks of the TGV line. On one track, blocks use alternately 1700 Hz and 2300 Hz, while on the other track blocks use alternately 2000 Hz and 2600 Hz. Upon these carrier frequencies can be modulated 27 separate audio frequencies, any combination of which can be present at one time. (The earlier TVM-300 uses 18 separate frequencies, only one of which could be present at any time.) Each block has a receiver at the opposite end from the transmitter, and the loss of the track circuit signal (due to shorting by train wheels or due to a failure) is interpreted as an indication that the block is occupied. Signaling block boundaries are equipped with electrical separation joints that prevent adjacent blocks from interfering with each other while letting the traction return current (at 50 Hz) pass through. (The technical designation is the UM71 track circuit.)
The signals which are present in the rail are detected by antennas mounted underneath the front airdam of TGV trains, about 1 metre (3 feet) ahead of the front axle. These antennas work by inductively coupling to the AC signal shunted between the rails by the first axle. There are four redundant antennas per train, two at each end. Only the two at the "front" of the train (in the direction of travel) are used. The signal from the track circuit is filtered, conditioned, and decoded onboard the train by two redundant digital signal processors.
The decoded signal takes the form of a 27-bit digital word, with each bit corresponding to one of the 27 frequencies encoded on the carrier frequency in the track circuits. This word contains several fields, in the following order:
Speed Codes containing three pieces of information: the current maximum safe speed in the block, the target speed at the end of the block, and the target speed at the end of the next block. Each of these can take on six different values; in the case of a high speed line these are (in km/h) 300, 270, 230, 170, 80 and 0, roughly corresponding to a typical deceleration profile.
Gradient information, averaged over the length of the block. This allows the train's signaling computers to account for this in speed calculations.
Block Length, which can vary quite a bit, and is also important in speed calculations. For example, on a flat stretch of high speed track, a block can be a full 1500 m (1 mile) long, while in the terminal areas of the Channel Tunnel, blocks are one-tenth as long.
Network Code, a number which determines the interpretation of the speed codes which should be taken by the train's computer. For example, on high speed lines where the maximum allowable speed is 300 km/h (186 mph), a different network code is used from that in the Channel Tunnel, where the speed limit is 160 km/h (100 mph). Eurostar trains need this information since they operate both on high speed tracks and in the tunnel.
Error-Checking code, allowing the integrity of the entire 27-bit word to be checked. If the information has been misread, the error can not only be detected from the error checking code, but can in some cases be corrected. The code takes the form of a 6-bit cyclic redundancy check (CRC).
These 27 bits of information are used as an input to the train's signalling computer, the on-board part of the TVM-430 system. In older versions of TVM, the target speed was updated only at every block boundary, resulting in a "staircase" style speed profile which is not representative of the continuous speed changes effected by the driver. However, with the additional information of block length and profile, TVM-430 is able to generate a continuously varying target speed through calculations performed in the on-board signalling computer, thus giving a much more realistic speed profile of contiuous acceleration or deceleration for the driver to follow.
In addition to the continuous speed control afforded by TVM-430, single instructions can be passed to the train by inductive loops located between the rails, which couple to a corresponding sensor under the train. Using the same frequency encoding principle, 28 bits of information can be recovered from a beacon, at speeds up to 400 km/h (250 mph). They come in two sizes depending on the line speed. They are 7 m and 4.5 m in length. These are called BSP(boucle sans ponctuel)',ITL's or Intermitent Transmission Loops. They consist of 2 half loops which transmit the message via a 125khz frequency, phase shifted with a 62.5khz carrier frequency. The information passed along concerns a variety of actions, such as
Indicating entry or exit from a high-speed line
Arming or disarming the TVM-430 system
Closing air conditioning vents before entering a tunnel
Raising or lowering pantographs
Switching supply voltages
A "black box" passive recording system watches over the entire process, monitoring a variety of parameters, not unlike the flight data recorders in aircraft. In TVM-430-equipped trainsets, the older graphical recording equipment has been replaced by the ATESS digital recording system. Every action of the driver (throttle, brakes, pantographs, etc) as well as signaling aspects (for TVM-430, KVB, and conventional signals) are recorded on tape for analysis using a desktop computer.
Another system, known as VACMA watches driver consciousness. It consists of a control that the driver needs to hold down for the TGV to move. It must remain depressed for a certain period of time. There is one period of time before a buzzer sounds, and another period of time before the automatic brakes come on. The control can be released for a very short period of time before a buzzer sounds, and another very short period of time before the automatic brakes come on.

What does the driver see?
In the centre of the desk in a TGV cab, just below the windscreen, there is a double or treble row of square indicators. This is where target speeds for the current and subsequent blocks are displayed to the driver, in the form of numbers (in km/h) on a colour-coded background. Full line speed is indicated in black numerals on a green background, while slower aspects are indicated in white numerals on a black background and a full stop is indicated as "000" on a red background. Below this display is the speedometer, where the continuously varying target speed is indicated as well as the current speed. (Speed is measured by a redundant tachometer to a precision of 2%.) The allowable variation between target speed and actual speed is dependent on speed, and is smaller at higher speeds. For an indication, under a 300 km/h aspect, the computer will only take action by applying emergency brake, if the train exceeds 320 km/h. Full details of the displays a driver may see are available at this site.
All the in-cab signaling displays must be very reliable, since they are critical to safety. They have relay-based position sensors which feed back to the signalling computer the current aspect being displayed to the driver. If there is a failure in the display unit, appropriate action is taken to stop the train.
In order to reduce stress on the driver, speeds are displayed over several blocks ahead of the train. When a block is followed by a more restrictive (slower) block, the display for that block flashes so the driver can better anticipate the speed change without releasing the brake. Restrictive indications can only be updated at block boundaries, except in emergencies. They are accompanied by an audible in-cab horn signal. Restrictions can however be lifted at any time within a block.
TVM-430 has extensive redundancy built into it, and one might wonder why it isn't used to control the train directly. However, in view of the lack of adaptability of the system to unexpected situations, it is considered desirable to retain a human in the loop. Driving a TGV is therefore done entirely manually, but the signalling system keeps a very close watch to ensure maximum safety.

Automatic Train Protection (ATP) System

Automatic Train Protection (ATP) in Great Britain refers to either of two implementations of a train protection system installed in some trains in order to help prevent collisions through a driver's failure to observe a signal or speed restriction. Note that ATP can also refer to automatic train protection systems in general, as implemented in other parts of Europe and elsewhere.
This system uses a target speed indication and audible warnings to warn the train driver if they are likely to exceed a speed profile that will cause the train to pass a red signal or exceed a speed restriction. The system will apply the brakes if the driver fails to respond to these warnings. The system takes into account the speed and position of the train relative to the end of its 'movement authority' in issuing the warnings and applying the brakes.
By the 1980s microprocessors had developed sufficiently for BR to carry out pilot trials on existing European ‘off the shelf’ ATP – fitting part of the Great Western Main line with the TBL1 system from ACEC and the Chiltern Main Line route with SELCAB a derivative of the German LZB system from Alcatel and GEC.
In the early 1990s, following the Clapham Junction rail crash in December 1988, and two other fatal accidents in early 1989 caused by SPADs, British Rail was keen to implement the ATP system across the entire British railway system. However, the cost (estimated at over £1bn) was baulked at by the Conservative government, who were preparing the company for privatisation.
All First Great Western's High Speed Trains (HSTs) are now fitted with ATP, and are not allowed to carry passengers unless the system is functioning. This requirement is in response to the Ladbroke Grove rail crash.
ATP is given permitted speed and location information from the track via encoded balise(s), encoded track circuit or more recently via radio.

European Train Control System

History

The specification was written in 1996 in response to EU Council Directive 96/48/EC of 23 July 1996 on the interoperability of the trans-European high-speed rail system. ETCS is developed as part of the European Rail Traffic Management System (ERTMS) initiative, and is being tested by six Railway companies since 1999. In Hungary, there is one ETCS Level 1-equipped line between Budapest and Hegyeshalom. ETCS Level 2 is used on the Rome - Naples line, opened in December 2005.
European railway networks grew as separate national networks that have little more in common than standard gauge. Notable differences include different voltages, signalling and control systems.
ETCS is divided up into different equipment and functional levels. The definition of the level depends on how the route is equipped and the way in which information is transmitted to the train. Basically, the movement authority (“permission to proceed”) and the corresponding route information are transmitted to the train and displayed for the driver in the cab ("cab signalling"). A vehicle fitted with complete ERTMS/ETCS equipment (EuroCab) and functionality can operate on any ETCS route without any technical restrictions.

Levels of ETCS

ETCS – Level 0
If an ETCS vehicle is used on a non-ETCS route this is known as Level 0. The trainborne equipment monitors the train for maximum speed. The train driver observes the national trackside signals.

ETCS – Level 1
ETCS Level 1 is a cab signalling system that can be superimposed on the existing signalling system, i.e. leaving the fixed signal system (national signalling and track-release system) in place. “Eurobalise” radio beacons pick up signal aspects from the trackside signals via signal adapters and telegram coders (Lineside Electronics Unit (LEU)) and transmit them to the vehicle as a movement authority together with route data at fixed points. The on-board computer continuously monitors and calculates the maximum speed and the braking curve from this data. Because of the spot transmission of data, the train first has to travel over the Eurobalise beacon in order to obtain the next movement authority. With the installation of additional Eurobalises ("infill balises") or a EuroLoop between the distant signal and main signal, the new proceed aspect is transmitted continuously. The EuroLoop is an extension of the Eurobalise over a particular distance which basically allows data to be transmitted continuously to the vehicle over cables emitting electrical radiation.

ETCS – Level 2
ETCS Level 2 is a digital radio-based signal and train protection system. Movement authority and other signal aspects are displayed in the cab for the driver. Apart from a few indicator panels it is therefore possible to dispense with trackside signalling. However, the track-release signalling and hence the train integrity supervision still remain in place at the trackside. All trains automatically report their exact position and direction of travel to the Radio Block Centre (RBC) at regular intervals. Train movements are monitored continually by the radio block centre. The movement authority is transmitted to the vehicle continuously via GSM-R together with speed information and route data. The Eurobalises are used at this level as passive positioning beacons or “electronic milestones”. Between two positioning beacons the train determines its position via sensors (axle transducers, accelerometer and radar). The positioning beacons are used in this case as reference points for correcting distance measurement errors. The on-board computer continuously monitors the transferred data and the maximum permissible speed.

ETCS – Level 3
In Level 3, ETCS goes beyond the pure train protection functionality with the implementation of full radio-based train spacing. Fixed track-release signalling devices (GFM) are no longer required. As in ETCS Level 2, trains find their position themselves by means of positioning beacons and via sensors (axle transducers, accelerometer and radar) and must also be capable of determining train integrity on-board to the very highest degree of reliability. By transmitting the positioning signal to the radio block centre it is always possible to determine which point on the route the train has safely cleared. The following train can already be granted another movement authority up to this point. The route is thus no longer cleared in fixed track sections. In this respect ETCS Level 3 departs from classic operation with fixed intervals: given sufficiently short positioning intervals, continuous line-clear authorisation is achieved and train headways come close to the principle of operation with absolute braking distance spacing (“moving block”). Level 3 is currently under development. Solutions for reliable train integrity supervision are highly complex and are hardly suitable for transfer to older models of freight rolling stock.

Main Railway Signalling Companies

Signalling and Communications
ACORDE - Broadband Railway Internet Access on High Speed Trains Via Satellite Links
Alcatel - Train Control, Rail Signalling and Communication Solutions
ALSTOM - Signaling and Train Control Systems
Ansaldo - Railway Signalling Systems
Banner Engineering - Industrial, Radar-Based Sensors for Train Detection
Bombardier Transportation - Rail Control Solutions
Cactus Automation - Traffic Management System
CDSRail - Remote Condition Monitoring of Railway Infrastructure
Central Electronics Limited - New Generation Railway Signalling Systems
CONDOR Signal & Communications - Switch Clearing Devices and Grade Crossing Solutions
Dialight - LED Lighting for Rail Signalling and Level Crossing Signals
ERICO - Rail Signal Bonds, Track Tools, Cable Clips and Rail Protection Devices
GE - Transportation, Global Signalling - Integrated Systems Solutions For The Rail Industry
Henry Williams - Manufacturer of Control and Signalling Equipment for Railways
Integra Hindustan Control - Audio Frequency Track Circuits (AFTC), Signalling Relays, Railway Signalling System
Kockum Sonics - Train Whistle Signals for Locomotives and Rolling Stock
Lumination - Railway LED Signals
McML Systems - Complete Signalling Systems
Nomad Digital - Mobile Data Communications Services for Trains
Prover Technology - Accelerating Development and Reducing Cost of Provably Correct Rail Control Systems
Railroad Signal International - Railroad Crossing Warning Signal Systems and Equipment
Sanarti Group - LED Signals, Warning and Signalling Lamps, Locomotive and Train Operation Safety Equipment and Railway Illuminations
Selenia Mobile - GSM-R Railway Communications
Siemens Transportation Systems - Signalling and Control Systems
TELEFUNKEN - Radio Communication Systems
Testing Installation and Correlation Services (TICS) - Signalling Design and Signal Works Testing
Thales - Specialist Telecommunications Services to the UK Rail Industry
The Specialty Bulb Company - Railway and Transit Lamps
Union Switch & Signal - Complete Rail Signalling and Control Solutions
Westinghouse Rail Systems - Signalling and Train Control Systems

Railway Signalling

Railway Signaling systems represent the earliest attempts of the modern society to improve the quality of one's life by technology.
The fundamental need for Railway signaling has been for the protection of human life the risk of which increases when it is moving using artificial means as Railway or any other automobile.

Railway transportation was among the first means to seriously increase speed of movement and the momentum of human beings. Therefore the Railway represents the first form of threat to human life due to movement at high speed . Railway signaling also represents one of the first use of technology for protection of human life.

Safety while maintaining speed with large loads requires use of technologies at their best.