SNCF automatic block light signals

Luminous Automatic Block Signal (BAL) is an automatic signaling system used in France by the SNCF to ensure the spacing and protection of trains running on the same track. It uses light signals placed at the entrance to each block, and track circuits that change their appearance as trains move along.

This system is used on high-traffic main lines, with a defined block length of between 1,500 and 2,800 m, allowing for high throughput.

Signals are made up of panels (targets) with a black background and white edging, bearing several lights. The main indications encountered on automatic blocks are as follows:

The square (CR or CV)

The red square indicates a stop and must never be crossed. It is a protective signal used to protect switches or parts of track (points, crossings, parking areas).

Equivalent to the red square, this signal commands a stop. The purple square is usually installed on service tracks.

The semaphore (S)

The semaphore commands a stop, and can be passed on sight after a complete stop in certain cases. It is a blocking signal, used to space trains apart and thus avoid overtaking or nose-to-nose.

The flashing red light is a variant of the semaphore, which allows you to proceed on sight without stopping, as long as you don't exceed 15 km/h when crossing the signal.

If the semaphore red light is displayed on a sign with an "Nf" plate, a small white light (called an "eyecup") must also be lit to confirm that what is displayed is indeed a semaphore, and not a square with one of the red lights burnt out.

The Warning (A)

The warning commands the driver to stop before the next signal. The warning may announce a signal displaying the red square/purple square/flashing red light, an accidentally extinguished signal or a knocker.

The flashing yellow light is used to announce a warning which is at a reduced distance from the stop signal, i.e. at a distance less than the normal braking distance.

Slowdown (R)

The 30 deceleration announces a switch (or several switches) in the "deviated lane" position to be passed at a maximum speed of 30 km/h.

Deceleration 60 indicates one (or more) points in the "deviated lane" position to be passed at a maximum speed of 60 km/h.

The slowdown reminder (RR)

The 30 deceleration reminder precedes a switch (or switches) in the "deviated lane" position to be changed to a maximum speed of 30 km/h.

The 60 deceleration reminder precedes a switch (or several switches) in the "deviated lane" position to pass at a maximum speed of 60 km/h.

The free way (VL)

The green light indicates to the driver that normal traffic is authorized, if nothing is in the way.

Equivalent to the fixed green light for trains whose speed limit does not exceed 160 km/h, the flashing green light commands the driver of a train with a higher speed limit to reduce its speed to 160 km/h before passing the next signal.

Walking in maneuver (FB)

The white light controls or confirms shunting. The driver must not exceed a speed of 30 km/h on switches and crossings.

The flashing white light controls shunting on a short route. In all cases, it prevents a train from departing in line.

Different panel configurations

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