Historically, each private railway company designed and used its own signals. However, during the First World War the interpenetration of trains between networks had increased, so that it became necessary to create a new unified signals specification. A commission was set up in May 1926, directed by Eugène Verlant of the PLM.

The Verlant commission submitted its report at the end of 1927. The new code of signals received the approval of the Ministry of Public Labour on 1 August 1930. Conversion to the Verlant code was completed only at the end of 1936, except on the network of Alsace-Lorraine where it was completed later, because of the unusual pre-existing signalling.

The Verlant code was very innovative, based on simple principles:

• Mainly based on color light signalling, which thereafter simplified the installation of the automatic block signalling.
• Light signals used three basic colors: red (stop), yellow (to announce stop or limited speed), green (clear). This color code was already applied by many foreign companies. It was also used for traffic lights.
• Simplification: it presented only the most imperative indications (except in special cases).

Signals are normally placed on the side of the line: on the left if the trains run on the left. However, in the Départements of Bas-Rhin, Haut-Rhin and Moselle, trains normally run on the right and so signals are placed on the right. This area was part of Germany from 1870 to 1918; trains run on the right in Germany and the railways were built during that period. When the area became French again in 1918, the railway network was almost complete, and trains ran on the right. Reversing the direction would be too expensive, in particular for interlocking, so circulation and signals remained on the right.

In several stations and on several line sections, when the local layout requires it, signals can be set on the right. They are then equipped with a white arrow indicating the way to which they are addressed.

On some double-track line sections equipped with "permanent counter-track installations" (IPCS), signals are placed normally on the right for counter-track circulations. No white arrows are used. In fact, entering the wrong way is confirmed by a luminous counter-track entrance board (TECS), which, when lit, indicates that the following signals are placed on the right (on the left for Alsace and Moselle). Similarly, a counter-track exit board (TSCS) indicates the return to normal running, with signals placed on the normal side.

Where trains run on the left, the hand-held stop signals (red flag, stop marker or lantern with red light) are presented on the left or at the center of the relevant track. If there is a platform by the track, they can be presented on the side of the platform.

In automatic block signalling, the signalling device is generally one of three categories:

• Non-permissive signals are absolute signals; they protect routes or points, or delimit sidings. Such signals can be recognized by their "Nf" (Non-franchissable, non-passable) plate. In their most restrictive aspect, they cannot be passed except under exceptional circumstances;
• Permissive signals regulate traffic to prevent trains from rear-ending the next one ahead. They bear plates such as "F" (Franchissable, passable) or "PR" (Permissivité restreinte, limited permissiveness). Non-permissive signals can also temporarily become permissive, in such case their "Nf" plate is accompanied with a small white light (Œilleton). The most restrictive aspect of a permissive signal allows the train to proceed after a certain number of conditions have been met.
• Distant signals only display deferred orders and require the driver to adapt their speed in order to be able to safely stop at the next point or signal. "A" (Avertissement, warning) or "D" (Disque, disk) plates are example of distant signals.

In general, the signalling device comprises:

• a 'block free' signal (green), meaning that the next block is clear or, in the case of a distant signal, that the next signal is also green;
• Warning signals or speed limits (yellow) requiring the driver to slow the train and especially to be able to stop before the next stop signal;
• Stop signals (red) require the train to stop.

These signals are supplemented by "indicator signals" (French: tableaux indicateurs) showing speed limits, slow-down orders and reminders to go slow on a diverging route, various indications about the track layout (number, dead ends or garage) the signs of electrical section, numbers of radio channels.

Mobile and temporary signals (e.g. construction), are used to complement permanent fixed signals.

Some signals are specific to manoeuvres. These include various types of signals:

• Hand signals,
• Mechanical signals,
• Lights,
• Acoustic signals
• Signal board.

Respect of signals is an imperative condition sine qua non of safety. The first section of the safety regulations of the SNCF indicates that "any official, whatever his rank must obey passively and immediately any signals that are presented."

The block system is based on breaking a line into block sections. The block sections on a line between two stations are part of the block system.

Non interlocked manual blocks: For safety reasons, non-interlocked manual blocks are used only on double track.

Interlocked manual blocks: In the case of a single track, interlocking is used in manual blocks to avoid an error resulting in a head-on collision.

Automatic blocks: Two types of automatic block are in common use in France. BAL (Bloc Automatique Lumineux) is used on high traffic lines with block lengths of about 1500 m) and BAPR (Bloc automatique à permissivité restreinte) is used in low traffic areas with block lengths up to 15 km. In both cases they are similar to the UK automatic block signalling.

Normally a train can only enter a block if it is free. The block is a track section delimited by signals, whose length depends on the distance needed for a train to stop or slow down, in the worst conditions on the portion of line under consideration. When traffic density is low, the blocks may be longer.

In future, the blocks may be mobile and follow the progress of the train (virtual blocks, not materialized on the ground and calculated continuously by an integrated onboard system). This will optimize the use of the line and reduce the distance needed between successive trains. This moving block system is already in use on the central section of the RER line A between Nanterre-Préfecture and Vincennes. The trackside signals are still in place but are turned off at the approach of a train whose mobile system signalling (SACEM) is confirmed in operation. They are turned on if problems arise.

The current signalling system has already reduced the spacing between trains from 3 to 2 min. But this is still too long on the busiest lines (suburban, and high-speed lines). In comparison, the moving blocks of the RER A allow a separation of only 90 seconds between trains at full speed.

Here are some examples of signals used on the French network.

Signals and signs give information and special instructions to train drivers. For simplicity, only the most common elements are presented here.

Stop signals and warning signals edit

Speed limits edit

Shunting edit

Stopping points in stations edit

Indications for electric locomotives edit

Other signs edit

With the arrival of the TGV, and its operating speeds well over 200 km/h, trackside signalling had to be abandoned in favour of cab-signalling, using track to cab communication technology known as Transmission Voie-Machine (TVM).

This system is intended to replace the different national signalling systems.

• Gernigon, Alain (1998). Histoire de la signalisation ferroviaire français. Paris: Editions La Vie du Rail. ISBN 2-902808-69-0.
• Lemon, R. (1995). "An Introduction to French Signalling". Signalling Record Society Signalling Paper. 13.
• "Les différents types de blocks automatiques à la S.N.C.F.". La Vie du Rail. 1534 (14 March 1976): 10–12.
• Wurmser, D (2007). Signaux Mécaniques. Grenoble: EPresses and Editions Ferroviairesl.
• "Les signaux – Les régimes d'exploitation des lignes – Les systèmes d'espacement des trains" (PDF) (in French). EPSF. 5 July 2017.

• French signalling
• French signalling section of The Signal Page
• The European Railway Signalling Server
• History of French signals
• Principles and description of SNCF signalling (in french)
• Speed control by beacons and SNCF in-cab signal repeaters (in french)
• SNCF signalling (in french)
• Biography of Eugène Verlant, history of code Verlant