Transmission Voie-Machine

Transmission Voie-Machine (TVM, English: track-to-train transmission) is a form of in-cab signalling originally deployed in France and used on high-speed railway lines. TVM-300 was the first version, followed by TVM-430.

TVM-300 was developed in the 1970s as part of the TGV project. At speeds of above 220 kilometres per hour, TGV trains run only on dedicated tracks designated as lignes à grande vitesse (LGV). At high-speeds it is not possible for a driver to accurately see colour-light based railway signals along the track-side. Signalling information is instead transmitted to the train and displayed as part of the train controls. The driver is shown the safe operating speed, displayed in kilometres per hour.

The 1980s-developed TVM-430 system transmits more information than traditional signalling would allow, including gradient profiles and information about the state of signalling blocks further ahead. This high degree of automation does not remove the train from driver control, although there are safeguards that can safely bring the train to a stop in the event of driver error.

The TVM system was developed by the French group Compagnie de Signaux et d'Entreprises Electriques (CSEE), now part of Ansaldo STS.

Two versions 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. Amongst other benefits, TVM-430 allows a train's onboard 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.

TVM-430 was presented from an intended "modular and flexible" range of signalling system levels from TVM-400 up to TVM-440 (optional automatic train control) and TVM-450 (full driverless control).

The line is divided into signal blocks of about 1,500 metres (~1 mi), 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 route. Trains at high-speed take several kilometres to stop. Since trains will require more than one signal block to slow down, drivers are alerted to reduce speed gradually, several blocks before any required stop.

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