France has a large network of high-speed rail lines. As of June 2021, the French high-speed rail network comprises of tracks, making it one of the largest in Europe and the world. As of early 2023, new lines are being constructed or planned. The first French high-speed railway, the LGV Sud-Est, linking the suburbs of Paris and Lyon, opened in 1981.
In addition to serving destinations across France, the high-speed rail network is also connected to the United Kingdom, Spain, Belgium, the Netherlands, Luxembourg, Germany, Switzerland and Italy. SNCF, France's state-owned rail company and the network’s main operator, runs both a premium service (TGV inOui) and a budget service (Ouigo). The French national high-speed rail network follows the spoke-and-hub model, centred on Paris. Besides SNCF, it is also used by Eurostar, Thalys, Deutsche Bahn, Trenitalia France, RENFE, and the Swiss Federal Railways.
Tracks
The newest high-speed lines allow speeds of in normal operation: originally LGVs were defined as lines permitting speeds greater than , revised to . Like most high-speed trains in Europe, TGVs also run on conventional tracks (), at the normal maximum speed for those lines, up to . This allows them to reach secondary destinations or city centres without building new tracks all the way, reducing costs compared to the magnetic levitation train project in Japan, for example, or complete high-speed networks with a different gauge from the surrounding conventional networks, in Spain and Japan, for example.
Track design High-speed railway track construction in France has a few key differences from normal railway lines. The radii of curves are larger so that trains can traverse them at higher speeds without increasing the centripetal acceleration felt by passengers. The radii of LGV curves have historically been greater than : new lines have minimum radii of to allow for future increases in speed.
LGVs can incorporate steeper gradients than normal. This facilitates planning and reduces their cost of construction. The high power/weight and adhesive weight/total weight ratios of TGVs allow them to climb much steeper grades than conventional trains. The considerable momentum at high speeds also helps to climb these slopes very quickly without greatly increasing energy consumption. The Paris-Sud-Est LGV has gradients of up to 3.5% (on the German NBS high-speed line between Cologne and Frankfurt they reach 4%). On a high-speed line it is possible to have greater superelevation (cant), since all trains are travelling at the same (high) speed and a train stopping on a curve is a very rare event. Curve radii in high-speed lines have to be large, but increasing the superelevation allows for tighter curves while supporting the same train speed. Allowance for tighter curves can reduce construction costs by reducing the number and/or length of tunnels or viaducts and the volume of earthworks.
Track alignment is more precise than on normal railway lines, and ballast is in a deeper-than-normal profile, resulting in increased load-bearing capacity and track stability. LGV track is anchored by more sleepers/ties per kilometre than normal, and all are made of concrete, either mono- or bi-bloc, the latter consisting of two separate blocks of concrete joined by a steel bar. Heavy rail (UIC 60) is used and the rails are more upright, with an inclination of 1 in 40 as opposed to 1 in 20 on normal lines. Use of continuously welded rails in place of shorter, jointed rails yields a comfortable ride at high speed, without the "clickety-clack" vibrations induced by rail joints.
The points/switches are different from those on the lignes Classique's. Every LGV set of points incorporates a swingnose crossing (coeur à pointe mobile or 'moveable point frog'), which eliminates the gap in rail support that causes shock and vibration as wheels of a train pass over the 'frog' of conventional points. Eliminating these gaps makes the passage of a TGV over LGV switches imperceptible to passengers, reduces stresses on wheels and track, and permits much higher speeds, . At junctions, such as the junction on the TGV Atlantique where the line to Le Mans diverges from the line to Tours, special points designed for higher speeds are installed which permit a diverging speed of .
The diameter of tunnels is greater than normally required by the size of the trains, especially at entrances. This limits the effects of air pressure changes and noise pollution such as tunnel boom, which can be problematic at TGV speeds.
Traffic limitations LGVs are reserved primarily for TGVs. One reason for this is that line capacity is sharply reduced when trains of differing speeds are mixed, as the interval between two trains then needs to be large enough that the faster one cannot over-take the slower one between two passing loops. Passing freight and passenger trains also constitute a safety risk, as cargo on freight cars could be destabilised by the air turbulence caused by the TGV.
The permitted axle load on LGV lines is 17 t, imposed to prevent heavy rolling stock from prematurely damaging the very accurate track alignment ('surface') required for high-speed operation. Conventional trains hauled by locomotives are generally not allowed, since the axle load of a typical European electric locomotive exceeds 20 t. The only freight trains that are generally permitted are mail trains run by the French postal service, using specially adapted TGV rolling stock and running at TGV speed. TGV power cars, the lightweight streamlined locomotives at both ends of TGV trainsets, are within the 17 t limit, but special design efforts were needed (a 'hunt for kilograms', chasse aux kilos) to keep the mass of the double-deck TGV Duplex trains within the 17 t limit when they were introduced in the 1990s.
The steep gradients common on LGVs would limit the weight of slow freight trains. Slower trains would also mean that the maximum track cant (banking on curves) would be limited, so for the same maximum speed a mixed-traffic LGV would need to be built with curves of even larger radius. Such track would be much more expensive to build and maintain.
Some stretches of less-used LGV are routinely mixed-traffic, such as the Tours branch of the LGV Atlantique and the Nîmes/Montpellier branch of the LGV Mediterranée. The British High Speed 1 from the Channel Tunnel to London has been built with passing loops to support freight use, but this facility is used infrequently.
Maintenance on LGVs is carried out at night, when no TGVs are running.
Outside France, LGV-type lines often carry non-TGV intercity traffic, often as a requirement of the initial funding commitments. The Belgian LGV from Brussels to Liège carries loco-hauled trains, with both the Dutch HSL-Zuid and British High Speed 1 planned to carry domestic intercity services respectively and international services. The Channel Tunnel is not an LGV, but it uses LGV-type TVM signalling for mixed freight, shuttle and Eurostar traffic at between . The standard pathway for allocation purposes is the time taken by a Eurotunnel shuttle train (maximum speed ) to traverse the tunnel. A single Eurostar running at occupies 2.67 standard paths; a second Eurostar running 3 minutes behind the first "costs" only a single additional path, so Eurostar services are often flighted 3 minutes apart between London and Lille. A freight train running at occupies 1.33 paths, at 3 paths. This illustrates the problem of mixed traffic at different speeds.
Train class Speed Paths Eurostar 160 km/h "catches up" with earlier trains Eurostar (second of pair) 160 km/h 1 consecutive "flighted pair" at same speed Eurotunnel Shuttle 140 km/h 1 optimal usage, all trains at same speed Multi-modal freight 120 km/h "holds up" train behind it
Power supply LGVs are all electrified at 25 kV 50 Hz AC. Catenary wires are kept at a greater mechanical tension than normal lines because the pantograph causes oscillations in the wire, and the wave must travel faster than the train to avoid producing standing waves that would cause the wires to break. This was a problem when rail speed record attempts were made in 1990; tension had to be increased further still to accommodate train speeds of over . On LGVs only the rear pantograph is raised, avoiding amplification of the oscillations created by a front pantograph. The front power car is supplied by a cable along the roof of the train. Eurostar trains are long enough that oscillations are damped sufficiently between the front and rear power cars (British designers were wary of running a high-power line through passenger carriages, thus the centrally located power cars in the ill-fated Advanced Passenger Train), so the two power cars could be connected without a high voltage cable through passenger vehicles. The same applies when two TGVs run in multiple. On lignes classiques, slower maximum speeds prevent oscillation problems, and on DC lines both pantographs must be raised to draw sufficient current.
Separation LGVs are fenced to prevent trespassing by animals and people. Level crossings are not permitted and overbridges have sensors to detect objects that fall onto the track.
All LGV junctions are grade-separated, the tracks crossing each other using flyovers or tunnels, eliminating crossings on the level.
Signalling
Because TGVs on LGVs travel too fast for their drivers to see and react to traditional lineside signals, an automated system called TVM, "Transmission Voie-Machine" (track-to-train transmission) is used for signalling. Information is transmitted to trains by electrical pulses sent through the rails, providing speed, target speed, and stop/go indications directly to the driver via dashboard-mounted instruments. This high degree of automation does not eliminate driver control, though there are safeguards that can safely stop the train in the event of driver error.
An LGV is divided into signal blocks of about 1500 m (≈1 mile) with the boundaries marked by blue boards with a yellow triangle. Dashboard instruments show the maximum permitted speed for the current block and a target speed based on the profile of the line ahead. The speeds are based on factors such as the proximity of trains ahead (with steadily decreasing speeds permitted 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 the LGV. As trains cannot usually stop within one signal block, which can range in length from a few hundred metres to a few kilometres, drivers are alerted to slow gradually several blocks before a required stop.
Two versions, TVM-430 and TVM-300, are in use. TVM-430 was first installed on the LGV Nord to the Channel Tunnel and Belgium, and supplies trains with more information than TVM-300. Among other benefits, TVM-430 allows a train's onboard computer 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 by displaying the Flashing Signal Aspects on the speedometer. When the flashing signal is displayed, the driver must apply the brake and target speed will be more constrained at the next block section.