Transmission voie-machine

Transmission voie-machine (French: [tʁɑ̃smisjɔ̃ vwamaʃin]; TVM; 'Track-to-train transmission') is a form of in-cab signalling originally deployed in France and is mainly 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 faster than 220 kilometres per hour, TGV trains only run along dedicated tracks designated as lignes à grande vitesse (LGV). When travelling at high speed, it is not possible for the driver to accurately see colour-light railway signals at the side of the track. Signalling information is instead transmitted to the train and displayed on the train driver's dashboard. The driver is shown the safe operating speed, measured in kilometres per hour.

The 1980s-developed TVM-430 system provides more information than traditional signalling systems would allow, including track 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 special safety mechanisms 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 Hitachi Rail 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 is also utilized on the KTX network in South Korea, further enhancing high-speed rail operations in the region.

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 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. A digital display on the train driver's dashboard shows the maximum permitted speed for a train's current block, as well as a target speed based on the status 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.

The signalling system is permissive; the driver of a train is permitted to proceed into an occupied block section without first obtaining authorisation. Speed in this situation is limited to 30 kilometres per hour (19 mph) 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 authorisation from the Signalling and Control Centre (Poste d'Aiguillage et de Régulation) before entering the block. 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. The driver then acknowledges the authorization using a button on the train's control panel. This temporarily overrides the emergency braking system, which would otherwise stop the train when proceeding past the non-permissive marker's ground loop.