Safety problems were solved by dividing the lines into sections called blocks, which can normally only be occupied by a single train. The entrance to the block is protected by a signal which remains closed for as long as the block is occupied. The signal management system is known as a block, and has undergone many changes over the years.

Today, there are three types of block in France:

• Manual Block (BM),
• The Automatic Luminous Block (BAL),
• The Automatic Block with Restricted Permissivity (BAPR).

Direction signs and speed limits are common to all three types of block.

The manual block (BM)

Historically, the Block Manuel was the first to be put into service, and it's also the most rudimentary. It is still used on lightly trafficked lines. It's economical because it has few signals, but it requires more staff to operate it. Each block, which can measure from 10 to 25 km, is under the responsibility of a guard who regulates traffic.

BM : Train spacing

When a train enters block 3, the guard closes the semaphore, preventing other trains from entering block 3, and hands the track back to the guard in block 2 after checking that the entire train has passed the signal (hence the importance of end-of-train signalling). A warning signal at the semaphore's stopping distance is also presented to the locomotive engineer to allow a following train to slow down and stop before the semaphore. This stopping distance depends on the authorized speed limit on the line.

Train spacing by manual block

BM : Protection of a dangerous point

Where two railroad lines meet, the risk of collision is always high. To prevent two trains from becoming entangled, a protective signal of the square type on the main tracks, or a violet square on the service tracks, is installed close to the point. This device is used to arbitrate converging traffic. A warning signal is installed at the stopping distance of the square. To ensure that a train does not engage the gauge at the switch point after braking for too long, a slip distance of no more than a few hundred meters is provided between the protection signal and the switch point.

Protection of a bifurcation by manual block.

BM : Delayed stop

In the interests of cost savings, the disc was introduced to replace the square-warning group. This single signal instructs the locomotive engineer to switch to running on sight and stop before the next signal or the first turnout encountered. As a result, it can be seen within braking distance of special sidings or turnouts leading to the service tracks of small establishments.

Deferred stop by manual block.

BM : Slow-down

The crossing of a switch or series of switches in the diverging position is rarely carried out at normal speed, as the risk of derailment is too great. In this case, mobile signals are installed, which slow trains down to 30 km/h when the switch is in the deviated position, and are cleared when the switch is in the non-deviated position. Two signs are required: the first announces the slow-down (the engineer immediately begins to slow down his train), the second marks the point where the instruction is normally executed (just before the point). Once the entire train has passed the point, the locomotive engineer can resume normal operation in accordance with the train operating manual.

Slowing down by manual block.

The luminous automatic block (BAL)

To cope with the ever-increasing traffic, two solutions were considered:

• The first, and most costly, was to multiply the number of tracks, as PLM(*) had done on the Imperial Artery.
• The second, slightly more economical solution was to reduce the length of the blocks, and therefore the spacing between trains.

This meant that semaphores were closer together, and the reaction time required to maneuver them was often too short to be entrusted to a guard.

This system was finally adopted when the automatic systems were deemed sufficiently reliable, and gave rise to the Light Automatic Block.
The Lighted Automatic Block requires the installation of a large number of signals, and is used on high-traffic lines.

The Paris - Lyon - Marseille (PLM), also known as the "imperial artery" because it was inaugurated by Louis-Napoléon Bonaparte in 1849, is the main artery of the French rail network.

BAL : Train spacing

With BAL, the length of the blocks is close to the stopping distance of the trains, i.e. 1500 to 2000 meters. The signals then serve in turn as semaphore and warning signals, as shown in the diagram below. When the train enters block 4, the entry signal closes, and the signal conditioning entry into block 3 gives warning information.

Train spacing by Automatic Luminous Block.

Following the increase in speeds on certain lines, the lengths of the blocks have become too short to allow a train to stop. The use of pre-warning and possibly slow-down 160 (only for lines capable of speeds in excess of 160 km/h) alleviates this problem by imposing successive slow-downs over several blocks before the stop signal. In the diagram below, a train has broken down in block 5. A train reaching the entrance to block 2 slows down to 160 km/h, then brakes again as soon as it enters block 3, finally stopping at the foot of the semaphore that closes block 5.

Spacing of trains by Automatic Luminous Block on tracks with a speed limit greater than 160 km/h.

In special cases, such as signals on steeply sloping tracks, the semaphore is non-stop to prevent the train from stalling. The semaphore's red light flashes, indicating that the train can pass it at low speed (less than 15 km/h).

Non-stop semaphore in BAL.

In BAL, targets are non-circular for all signals. The square has a target with a minimum of 4 lights. It can cover the functions of a square with two red lights on, or a semaphore with a single red light on. Unlike other types of block, the Lighted Automatic Block is permissive: after stopping at the foot of a semaphore and recognizing the signal, the engineer can pass it on sight.
However, a semaphore must not be confused with a square with a burnt-out lamp. A passing eyecup, a small bluish-white light to the left of the target, is lit when the semaphore can be passed. Originally, all BAL signals had an eyecup, whether they were squares or semaphores. As a cost-saving measure, the eyecup was then replaced by a reflectorized plate F for "passable" on signals with only one semaphore (3-light target), with the exception of semaphores located on cradles hung from gallows, as their identification plate is more difficult to observe at night.

BAL : Protection of a dangerous point

In the diagram below, a square is set up to protect a bifurcation. This configuration is similar to that of the Manual Block. However, in the case of the BAL, the square can also be used as a spacing signal, prohibiting penetration of the next block as a simple semaphore would do. The signal can therefore play the role of a protection or separation signal, and the usefulness of the eyecup becomes fully apparent in this case.

Protection of a bifurcation by automatic luminous block.

BAL : Slow-down

Two types of slow-down are possible in BAL, for example, when crossing switchbacks in a diverted position: slow-down 30 and slow-down 60. The arrangement of the yellow lights is the same, but they flash for slowing down to 60 km/h.

Slowing down to 30 km/h when approaching a needle in deviated position.

Slowing down to 60 km/h when approaching a needle in deviated position.

When the switch is not in the deviated position, there is generally no need to slow down, as shown in the diagram below. In these conditions, the signals give the usual indications useful for the proper spacing of traffic.

Situation when approaching a needle in an undeviated position.

The automatic block with restricted permissiveness (BAPR)

The Automatic Block with Restricted Permissiveness is the latest block model to have been put into service in response to economic imperatives. It incorporates the automatic functions of the Lighted Automatic Block, but is applied to longer blocks. It therefore requires fewer signals than the BAL and fewer personnel than the BM. It is mainly used on medium-speed and secondary lines.

Why not simply retain the BAL principle and lengthen the townships?

The reason is obvious when you consider that a train encountering a warning must slow down and proceed as quickly as possible on sight to the next stop signal. Let's consider the following fictitious situation in a long-block BAL: a following train catches up with the train ahead. A warning is given, and the train is required to run on sight (i.e. at 30 km/h maximum) for the whole block, which is inconceivable for blocks of around ten kilometers (it would take 15 to 20 minutes to cover it!). Consequently, to avoid excessively long sightseeing, the semaphore in BAPR cannot be crossed - hence the term "restricted permissiveness" - and trains are spaced according to a very different scheme from that of BAL.

BAPR : Train spacing

The semaphore provides train separation. The warning signal that precedes it should not be confused with a disc. In fact, it bears an A identification plate. The diagram below shows a situation where blocks 1 and 3 are occupied. Their entrance is closed by a semaphore, and the preceding warning signal is also closed.

Train spacing by Automatic Block with Restricted Permissivity

BAPR : Protection of a dangerous point

Dangerous points such as junctions are protected by a square, or by a purple square if on service tracks. From this point of view, the layout of the signals is comparable to that of the Manual Block, but in this case, signal management is entrusted to a PLC that uses a track occupancy detection system.

Protection of a bifurcation between two lanes by BAPR.

Protection of a junction with a service road by BAPR.

BAPR : Slow-down

Slow-down 30 on approaching turnouts or crossings is identical for BAPR and BM. Only the automatic management system distinguishes them.

Slowing down to 30 km/h near a needle in deviated position.

Direction indication

The Direction Indicator Panel (TID) is located above the protection signal at a major junction, and consists of as many white lights as there are possible routes beyond the signal. The number of lights on corresponds to the number of the direction from the left (1 light = leftmost direction, 2 lights = 2nd direction from the left, etc.).

Example of the use of direction indicator panels. The machine is heading in the middle direction (second from left). Both white TID lights are on, even though the first switch points the train to the left-most track before the grade separation.

Speed ​​Limit

The Train Running Booklet (L.M.Tr.) informs the locomotive engineer of speed limits along the entire line. However, speed may be limited at dangerous points such as switchbacks, curves and tunnels. The speed limit is displayed on the trackside by fixed or mobile signs or light boards.

Example of a speed limit of 90 km/h in a defined zone.

Possible use in model railroading

• Realization of targets corresponding to the light signaling of automatic block of the SNCF

Source of documentation: Excerpt from Abrégé de signalisation SNCF (Emmanuel Bournez)