Main characteristics of
BB 25500 electric locomotives

BB 25500 locomotives were manufactured between June 1964 and January 1976 (194 locomotives built). Contemporary of the CC 6500 locomotives, they are bi-current locomotives 1500 V DC / 25000 V AC 50 Hz, and developed on the basis of older BB 16500 series, more precisely the BB 20004 prototype made from the BB 16540. The BB 16500 had been developed by Fernand Nouvion, chief engineer of the DETE (Electric Traction Studies Division) and put into service from 1958. The BB 25500 took back most of the technical design of BB 16500.

They are equipped with two single motor bogies, with two gear reduction ratios, making them mixed locomotives (freight or passenger).

First to come in 1964, BB 25500 gave way to several other series :
- BB 8501 to 8646 for 1500 V DC ;
- BB 17001 to 105 for 25 KV AC 50Hz mainly used in Northern and Western France ;
- BB 20201 to 20213 bi-frequency 25 KV 50 Hz - 15 KV 16 2/3 Hz based in Strasburg for connection with Switzerland (CFF) and Germany (DB).

Main characteristics of BB 25500 locomotives :

Manufacturers :

- Alsthom : body, bogies, traction motors, transformers, electric equipment ;
- CEM : variators.

Total length :
- 14,700 m / 48.23 ft. for 25501 to 25544
- 14,940 m / 49 ft. for 25545 to 25587
- 15,570 m / 51 ft. for 25588 to 25694

Mass :  80 metric tons

Continuous power (same power under 1500 V DC and 25 KV AC) : 2940 kW / 4000 HP

Max. speed in service : 100 km/h /62 MPH  (low speed gear ratio) 140 km/h / 87 MPH (high speed gear ratio)

Voltage : 1500 V DCand 25 kV 50 Hz AC.

Above : the BB 25605 of Strasburg depot is about to stop in Sanry-sur-Nied station while pulling a train from Rémilly to Metz for the Lorraine TER (Regional Express Trains) activity. It seen here under 25 kV 50, with corresponding pantograph raised. It bears figure 8, meaning that it was previously used in the Paris suburban services.
(Photo Pierre Chavernac)

Mechanical part


BB 25600 locomotives as well as BB 25500, BB 8500/8600, BB 17000 and BB 20200 are the direct descendants of the BB 16500. It is therefore logical that their body looks quite similar excepting a few details.

The chassis bears two reinforced driving cabs, side panels assembled to it by welding and roof elements bolted. The latter are removable for maintenance purposes like big components replacement : compressor, transformers, etc.

The two driving cabs are soundproof and equipped with a functional driver desk similar to the ones of BB 25200 and CC 6500, and an air heater.

The BB 25588 to 25694 (as well as BB 8588 to 8646) are equipped with stretched driving cabs (known as « large cabs ») in opposition to BB 25501 to 25587, 8501 to 8587 et 17000 fitted with small cabs.
Driver benefits of an adjustable seat bolted to the floor while assistant only has a simple seat on the floor, similar to the one of CC 6500 locomotives.

Driving cabs are not equipped with a front chock absorbing system. However, following an accident, the BB 17005 was equipped with special “nose” with such system. This modification was not extended to the other locomotives.

The two front glasses are vertical which doesn’t contribute to reduce glare in opposition to CC6500 inclined windshields. The windshield on driver side is equipped with an anti-fog system.

Left :

The BB 25585 of Vénissieux depot is seen close to Lyon-Perrache station. It shows the nice "multiservice" livery (but dark grey is a bit faded on this one).

On the BB 25588 to 694, body is longer, allowing larger driving cabs. This difference is materialized by the additional window between cab door and front end (see picture below).

(Photo APCC 6570)

Opposite :

The BB 25635 "large cab" of Marseille depot, in “concrete” livery, seen in Avignon depot.

Notice the two different pantographs which equip the BB 25500 :
- AM-18-U for AC single phase current on side 1 (left on the picture) ;
- AM-18-B for DC current, on side 2 (right on the picture).

The 25588 to 25694 have stainless steel vertical louvers on the whole body length.

(Photo Pierre Chavernac)


The very good grip results obtained with the BB 16500 single motor bogies, thanks to their much reduced wheelbase and mechanical coupling of the axles, have encourage designers to carry on with this bogie type on the BB 8500/8600, 17000 and 25500/25600. While grip is excellent due to the reduced when base (which reduces the pitch up effect in traction phase), stability was often criticized by drivers who gave to these locomotives series the nickname of “dancers”.

The bogie chassis is made of a welded assembly in which is placed the TAB 660 motor.

Each bogie is linked to the body by two drawbars.

Braking is done by a brake cylinder (one per bogie) acting on a brake rigging which itself acts on iron shoes applied on wheels (two per wheel).

The whole bogie chassis is placed on two beams with helical springs in between, making the primary suspension. The beams are linked to the axles, equipped with conical roller bearings, by an assembly of balls and rods.


The body is placed on bogie chassis with a load-bearing crosspiece equipped with helical springs giving a secondary suspension.

This load-bearing crosspiece is placed on the bogie via two conical supports (BB 25501 to 25555). On the BB 25556 to 25694 (sketch opposite), body is directly placed on the bogie through four “sandwich” blocks.



Primary suspension :

As mentioned above, primary suspension between axle box and bogie consists in two large helical springs inserted in two beams. Parasite movements are diminished by oil shock absorbers.

Secondary suspension :

Body is mounted on the bogie chassis with a load-bearing crosspiece (BB 25501 to 25555) or with four  “sandwich” blocks on 25556 to  25694.

Two anti-yaw dampers (two per bogie) as well as a transversal damper (one per bogie) ensure bogie and body stability.

(Opposite : SNCF document)




Traction motor is coupled to an external double ration gear reducer with a flexible Citroën coupling. Traction effort is transmitted from reducer to axles by a floating ring drive system.

Traction effort transmission to the axles goes through eight gears allowing using the locomotive with two different speed ratios.

Lubrication of this system requires a pump driven by one of the gears. This pump takes lubricant in the reducer casing and distributes it by flowing and injection in the bearings. Part of lubrication is made by splashing.



Each bogie includes a double ratio gear reducer on top of the bogie. Ratio changing action is manual and can only be done when locomotive is stopped, with main breaker open and brakes on.

Ratio changing action is done in the side corridor by opening a door on the floor giving access to motor coupling and reducer ratio switch control.

Locking system is actioned by a crank on the corridor wall. An index which first shows V (verrouillé : locked) is brought on D (déverrouillé : unlocked) by rotating the crank.
A metal bar is then inserted in the tipper to move it (position GV : High speed or PV : low speed). Tipper position is indicated on the outside of the bogie by an index moving on M (Marchandises : Freight or low speed) or V (Voyageurs : passengers or high speed).

(Opposite: SNCF document)


A neutral position (PM : point mort : Neutral) also exists. It is used in some particular cases mentioned in the troubleshooting guide, allowing to decouple the motor from the gear reducer in case of a mechanical problem on the tipper or the motor. Neutral position is also used during maintenance in order to rotate motor and tipper gears without moving the locomotive. This to detect abnormal noises on the gears bearings for instance, or to grind traction motor commutator.

During ratio switching, it may happen that gears teeth get face to face and don’t engage.
A bar is then used to manually rotate motor armature to put gear teeth in proper position.




Opposite left : A bogie seen on the reducer side. Suspension “sandwich” blocks are well visible too on the picture.

(Photo Victorien Armand)



Reducer to axles transmission

Reducer is coupled with axles with a floating ring drive, linked, on one side to the reducer and on the other side to the corresponding wheel to drive. This floating ring drive includes silent blocs and tie rods bolted on one of the two wheels of an axle. Bogie can thus moves freely following track imperfection but still transmitting traction effort.



BB 25600 have one-piece wheels and axle boxes  with conic rollers bearings. Axle boxes are equipped with lubricators to allow regular lubrication as well as temperature sensors to detect an axle box overheat. If this happens, a CB warning for Axle box overheating appears on the dashboard.
Shunts placed outside ensure traction current return.


Traction circuits

The BB 25600 are equipped with two traction motors TAB 660 giving 1470 kW / 2000 BHP.  These are classical DC traction motors with a field and an armature with its commutator. They are forced-ventilated.

The BB 25600 are equiped with a variator driven by a four cylinder pneumatic actuator. 32 positions can be used.

The two direction inverters, the two traction/braking switches and the two AC/DC switches can be actuated manually in the event of a problem on their electro pneumatic drive.


Traction under 1500 V DC :

Under DC current, series and parallel connections can be used with 32 variator positions.


1°) Series connection

Series connection allows starting a train with a maximum 750 V voltage on the motors. At the end of series connection, variator is on position 20.

With main DC breaker closed, when the driver accelerates, variator actuator rotates position by position, eliminating resistances between power supply and the motors.  The two motors connected in series are supplied through the rheostat. Progressive elimination of the resistances is made with electro pneumatic switches. At position 20, the rheostat is totally eliminated and voltage at each motor is 750 V.

 5 bridging positions are available, allowing increasing or keeping train speed.


2°) Parallel connection

Parallel connection is obtained by pushing forward the briding lever. At the end of progression under parallel connection, variator is on position 32.

When parallel connection is selected, part of the rheostat is reinserted. Driver then passes variator positions and when reaching position 32, rheostat is totally eliminated. Each motor then gets 1500 V equal to the overhead line voltage.
5 bridging positions are available again.

Variator positions were initially shown on the control panel (like on CC 6500). With KVB (speed control by sensors) installation, this was suppressed. Only an “RH” indicating light remain to let driver know that rheostat is not totally eliminated. This light comes off when variator is on position 20 (last series connection position) or position 32 (last parallel connection position).

Rheostats are cooled by three fans connected in parallel on a rheostat resistance. They therefore start once current crosses the rheostat and their rotation speed depends on this current.
The greater the current is, the higher the fans rotation speed is. Fans stop when variator is on position 20 or 32 (rheostat eliminated).

Traction under 25 kV 50 Hz AC :
(Sketch opposite)

Under AC current, on parallel connection is used. 32 positions are used.

Current taken from overhead line is supplying the primary winding of an autotransformer through single phased AC breaker. This autotransformer has 32 connections on its windings. Each position change lead to a switch from one connection to the next one. This is done with a oil insulated variator.
Position switching by the driver increases voltage on the secondary winding of the autotransformer by 750V. Outputs of the autotransformer supply the primary winding of the traction transformer.
The two secondary windings of this transformer supply respectively one traction motor through a diode bridge rectifier.

When on position 32, driver has 5 bridging positions available.

Under 25 kV AC, all variator positions are mentioned as “economical”. As a matter of facts, autotransformer and transformer efficiencies are excellent and only a low amount of energy is lost (in opposition to the rheostat which dissipates under heat form several thousands of kW.).
Driver can therefore use continuously any position of the variator without issues, in opposition to the rheostat which can, in the worst case scenario, melt if driver keeps variator on a rheostat position.

Autotransformer is equipped with a “heating” output to deliver needed current for passenger trains heating i.e. 1500 V AC 50 Hz. This output also supplies, through a diode bridge rectifier, main air compressor and traction motors fans.

Primary winding of auxiliary transformer is also supplied by heating output. Auxiliary transformer supplies at adequate voltage, the main transformer oil pump, rheostat fans and battery charging.

The three rheostat fans are on once AC breaker is closed to create an air circulation to cool main transformer cooling fluids.

BB 25600 can be coupled in multiple units for heavy trains pulling on difficult profile lines.
Multiple unit coupling is made through three cables in front of each locomotive. The same cables are used to drove the locomotive from the end cab of a reversible train like RIO or RRR types.

Anti-skid system :

Skid detection is done by comparing intensities absorbed by each motor. Once skid is detected, a light comes on on the control panel, variator actuator stops its progression and goes back of two positions. Sanding is automatically started. A motor armature shunting is done on the motor affected by skidding.

High voltage auxiliaries :

Battery charging is done under 1500 V using current return from motor fans and compressors.
When compressor or motor fans start, absorbed current crossed the batteries to charge them.
A thyristor system allows deviating part of the current in a resistance when batteries charge is sufficient to avoid an excessive water consumption.
When fans and compressor are off, a tension divider bridge ensures a 5 A permanent charging once DC breaker is closed.
If batteries voltage goes below 55V, a voltage relay automatically starts the fans.

Under 25 kV AC, batteries charging is done with a simple transformer.

These systems ensure main battery 72V charging as well as 9V braking battery.

Dynamic braking:

BB 25600 are equipped with dynamic braking alone or combined with pneumatic braking, as well as emergency dynamic braking. Driver can use 16 levels of dynamic braking.

Various equipment:

BB 25600 are equipped with following devices :

• One AC single phase circuit breaker DBTF ;
• One DC circuit breaker JRT ;
• Two pantographs : AM 18 B (1500 V) and AM 18 U (25 kV) ;
• One compressor and two air tanks underneath the body ;
• One auxiliary compressor ;
• One braking panel PBL2.

Text : Victorien Armand

BB 25660 control panel. Its structure is quite comparableto the CC 6500 one.
Difference is however the two pantographs controls (DC and AC) on the right.

(Photo Roland Badosa).

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BB 25500 series : service done with SNCF
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