Transmission voie-machine
At speeds faster than 220 kilometres per hour, TGV trains only run along dedicated tracks designated as lignes à grande vitesse (LGV).
The TVM system was developed by the French group Compagnie de Signaux et d'Entreprises Electriques (CSEE), now part of Hitachi Rail STS.
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.
[1][2] The line is divided into signal block sections of about 1,500 metres (~1 mi), the boundaries of which are indicated by blue square signs printed with a yellow-on-white triangle.
The signalling system is permissive; the driver of a train is permitted to proceed into an occupied block section without first obtaining authorisation.
Once a route is set, or the signalling centre has provided authorisation, a white lamp below the marker board is lit to inform the driver.
This temporarily overrides the emergency braking system, which would otherwise stop the train when proceeding past the non-permissive marker's ground loop.
When trains enter or leave the LGV from lignes classiques, they pass over a ground loop which automatically switches the driver's dashboard indicators to the appropriate signalling system.
It is one of the more advanced railway signalling systems in the world,[citation needed] although the implementation itself is still based on relays rather than solid state electronics.
Both run using Motorola 68020 class processors, and are programmed in Ada, a computer language often used in safety critical systems.
The ground-based segment of TVM-430 resides in trackside boxes, which control stretches of track about 15 km (9.3 mi) long.
The resulting word contains several fields, in the following order: These 27 bits of information are used as an input to the train's signalling computer, the onboard part of the TVM-430 system.
These are called BSP (boucle à saut de phase) Intermittent Transmission Loops (ITL).
In the centre of the driver's desk in a TGV cab, just below the windscreen, there is a double or triple 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 kilometres per hour) on a colour-coded background.
They have relay-based position sensors which feed back to the signalling computer the current aspect being displayed to the driver.
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.
Every TGV train is equipped with the KVB (Contrôle Vitesse par Balise, "Beacon Speed Control") which is used throughout the French ligne classique network.
