The TGV Signaling System
The signaling of a high speed line requires a different approach from conventional railways. The speed of the trains is high enough that the engineer/driver cannot reliably read signals placed at trackside. The required vigilance cannot be expected of a human, especially for long periods and in adverse weather conditions. This is why the TGV system relies exclusively on cab signaling, a system by which signaling information is transmitted through the rails as electrical signals which are picked up by antennas placed under the train. This information is then processed by computers and displayed to the engineer/driver in the cab.
Cab signaling is not a new concept and is commonly used around the world for speeds above about 160 km/h (100 mph). The type of cab signal used on the TGV system is TVM, which stands for Transmission Voie-Machine or "track to train transmission". It should be noted that as futuristic as TGV trains may appear, the engineer/driver in the cab is still fully in charge of the driving task.
The TVM system was developped by the French group CSEE. It uses track circuits in both rails to transmit signaling information to the train's on-board computers, as well as fixed inductive loop beacons. 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 antequated components, for example... relays!
TVM is a fixed block system: the track is subdivided into fixed segments each of which has a particular state. Only one train may occupy any block at one time under normal operation. On the high speed lines TVM is permissive, in that a train may proceed at reduced speed (visibility allowing) after having been ordered to stop.
TVM 430 is the cab signaling system used on the latest TGV lines, and is an evolution of the earlier TVM 300 system, which operates on similar principles. As of 1996, it is used on the Nord-Europe line (LN3), part of the Rhône-Alpes line (LN4), and in the Channel Tunnel. It will be installed on the Sud-Est line.
TGV lines are divided into fixed blocks about 1500 m (1 mile) long. (The earlier TVM 300 system uses longer blocks.) Blocks are shorter than a train's braking distance, so a braking sequence takes place over several blocks, nominally four. This relatively frequent subdivision allows to run trains on shorter headways, which increases the capacity of a high speed line without placing additional requirements on the braking performance of the trains. Minimum headways (time between two succesive trains) are 5 minutes on the Sud-Est line (which uses TVM 300), 4 minutes on the Atlantique line (also TVM 300) and just 3 minutes on the Nord-Europe line, which is equipped with the newer TVM 430.
The following table illustrates the combined effects of speed and block length on braking distances and train headways.
Max Speed (km/h) 270 300 300 360 Nominal Block Length (m) 2100 2000 1500 1500 Normal Braking Profile (m) 8400
Minimum Headways (mn) 5 4 3 3
(*) Note: some TGV Atlantique trainsets are fitted with TVM430
Blocks boundaries are indicated visually at trackside by square metal boards, blue with a horizontal yellow triangle (such as seen in the heading to this article). These signs are not critical to the task of driving a TGV, and they need not be seen by the engineer/driver.
Each block has certain properties which are relevant to the train occupying it. Invariant properties of a block are its length, its profile (uphill, downhill, flat), and a rated maximum safe speed, which is usually 300 km/h (186 mph). Properties that can change depending on the presence or absence of trains or other obstacles ahead are the target speed at the end of the current block, and the target speed at the end of the following block. A target speed is the speed at which the train should exit the current block and enter the next.
All of these pieces of information are relayed by the TVM 430 system to the train's computers and the cab display. The engineer/driver's responsibility is to follow the signal aspects indicated to him, but if he fails to do so he is closely watched by the train's computers, which can bring the train to a safe stop.
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 center, 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 meter (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
- The speed codes contain 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 five different values; in the case of a high speed line these are (in km/h) 300, 270, 230, 170 and 0, roughly corresponding to a typical deceleration profile.
- The gradient information is averaged over the length of the block. This allows the train's signaling computers to account for this in speed calculations.
- Block Length
- The block length 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
- The network code is 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 than 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
- The error checking code allows to check the integrity of the entire 27-bit word. 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 signaling 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 engineer/driver. However, with the additional information of block length and profile, TVM 430 the train is able to generate a continuously varying target speed through calculations performed in the on-board signaling computer, thus giving a much more realistic speed profile of contiuous acceleration or deceleration for the engineer/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). 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 passive recording system watches over the entire process, monitoring a variety of parameters, not unlike the "black box" 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 engineer/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.
What Does The Engineer/Driver See?
The image above shows the control desk of a TGV Duplex. In the center of the desk, just below the windshield, there is a double row of square indicators. This is where target speeds for the current and subsequent blocks are displayed to the engineer/driver, in the form of numbers (in km/h) on a color-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 take action only if the train exceeds 315 km/h.
A very complete description of TVM signal aspects and braking sequences was prepared by Thierry Davroux, and is well worth the visit. It is only available in French. The different speed codes, as displayed to the engineer/driver, are explained in detail.
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 signaling computer the current aspect being displayed to the engineer/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 engineer/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 engineer/driver can better anticipate the speed change. 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 signaling system keeps a very close watch to ensure maximum safety.
Additional Signaling Systems
The TVM system is used only on high speed lines in France. Outside of the high speed lines, other signaling systems are used, and every TGV train is equipped with them. KVB (Contrôle Vitesse par Balise, or "beacon speed control") is used throughout the French standard network and is therefore present on all TGVs. In addition to TVM, then, the following systems are used in various combinations:
- KVB the French signaling system (electro-mechanical with radio beacons)
- ATB the Dutch signaling system (induction based)
- ATB-NG a newer version of ATB (also induction based)
- MEMOR the Belgian signaling 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 signaling system (induction based)
This can make for some rather complicated signaling displays, especially on the new 4-system Thalys TGVs which are designed for international service.Last modified: Thu Apr 23 08:54:27 PDT 1998