French Railway Signalling - The Block System

The Block System

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

Non interlocked manual blocks: For security 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 in high traffic lines with block lengths of about 1500m) and BAPR (Bloc automatique à permissivité restreinte, used in low traffic areas with block lengths up to 15km)

A train can only normally enter a block if it is free. The block is a track section delimited by signals, whose length is a function of distance needed for stopping or slowing a train, in the worst conditions on the portion of line under consideration. When traffic density is low, the blocks may have a longer length.

In signalling systems of the future, the blocks will be mobile and follow the progress of the train (virtual blocks, not materialized on the ground and calculated continuously by an integrated on board system). This will optimize the use of a line and shorten the distance 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 is turned off at the approach of a train whose mobile system signalling (SACEM) is confirmed in operation, but turned on if problems arise.

The current signalling system, has already reduced the spacing between trains from 3 to 2 min. However, this is 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 90s between trains at full speed.

In general, the signalling device comprises:

• a 'go' signal (green), meaning that the next block is free, allowing normal operation
• 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) requires the train to stop.

If the distance between the stop signal and a distant signal is too short (it can sometimes get down to 400m), then the previous distant signal has a yellow flashing to inform the driver of the short distance between the two following signals.

These signals are supplemented by "indicator signals (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. "