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)
|Nominal Block Length (m)
|Normal Braking Profile (m)
|Minimum Headways (mn)
(*) 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
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
- 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
- The gradient information is averaged over the length of the block.
This allows the train's signaling computers to account for this in speed
- 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
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
- 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
- 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
Last modified: Thu Apr 23 08:54:27 PDT 1998