LGV signalling

This article was originally based on material from TGVweb, which is licensed under the GFDL.

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, 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 ten times shorter.
• 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 7m and 4.5m in length. These are called BSP(boucle sans ponctuel)',ITL's or Intermitant 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 (in French).

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.

Other signalling systems

The TVM system is used only on high-speed lines and in the Channel Tunnel. Outside of the high-speed lines, other signalling systems are used. Every TGV train is equipped with the KVB (Contrôle Vitesse par Balise, or "beacon speed control"), which is used throughout the French ligne classique network. In addition to TVM, then, the following systems are used in various combinations:

• KVB, the French signalling system (electro-mechanical with radio beacons)
• ATB, the Dutch signalling system (induction based)
• ATB-NG a newer version of ATB (also induction based)
• MEMOR, the Belgian signalling system (electro-mechanical)
• TBL, a newer version of MEMOR (electro-mechanical with radio beacons)
• InduSi the German signaling system (induction based)
• LZB, the German system for high-speed lines (also induction based)
• AWS, the British signalling system (induction based)
• TPWS, the warning system which supplements AWS

This can make for some rather complicated signalling displays, especially on the TGVs designed for international service, such as the Thalys PBKA and Eurostar trains.