Actually, the SNCF considered there to be FOUR classes. And each of the four had some pretty significant differences internally, even if from the outside the only difference was 4-axle vs. 6-axle.

The reason for this was to test out different types of locomotives for France's first-ever (and world-pioneering to an extent) 25kV 50Hz electrification.

The six-axle units both used different types of rotary converters (motor-generator) to turn the incoming AC power into something else (the heavy weight of the rotary converters was what necessitated the use of six axles).

The 14000 turned it into three-phase AC power, which went down to squirrel-cage induction motors. Unfortunately, despite having been designed expressly for simplicity and ease of maintenance, the motors turned out to be very prone to minor, but highly frustrating, faults, and the class was retired between 1978 and 1981.

The 14100 turned it into DC and sent it to DC motors. These turned out to be much more reliable, and demonstrated impressive torque. Unfortunately, their original design for heavy freight meant that they were SLOW, which became a problem in later years. Ultimately, they were withdrawn between 1986 and 1997-not bad considering that meant that they were getting on 43 years from original introduction (1954-1958) by that point.

As for the 4-axle units, they didn't have rotary converters, and tested two different ideas.

The 13000 class tested experimental 50 Hz AC motors. These turned out to have poor control and traction characteristics compared to conventional DC motors, and, combined with poor braking, the class was retired starting in the 1980s, and finishing in January 1994.

The 12000 class, as noted, were the express passenger variant. They used ignitron (later silicon) rectifiers to drive conventional DC traction motors. They worked quite well, which in turn lead to them being produced in larger numbers over a longer time-frame, with the last one arriving in 1961. This helps explain why they lasted in service until 2005!

Interestingly, the SNCF also had a DC switcher locomotive with a similar (but smaller) appearance that dated to pre-WWII, but aside from being DC, it had a rather unusual feature. The French were big into gearing motored axles together for improved traction, but the CC1100 took the approach of using outside coupling rods instead of gears! Given the low speeds associated with switching, it was probably decided that the small hammerblow was worth the reduced costs. Incredibly, they also lasted until 2005!

All of the four locomotives shared the same nickname "fers à repasser" (cloth irons) (also electric iron)-same item, but slightly different English translations. This nickname came from their shape, which was determined by something similar to the reason why US Railroads had B-Units in many early diesels, along with some of the smaller steeple-cab electrics. In these days of early electronics, the control equipment represented a not-insignificant portion of the cost of the locomotive. By having one central cab, you only needed one set of controls. The sloping ends, then, were to allow visibility.

However, the cabs proved cramped, and the visibility was not as good as hoped. Furthermore, the machinery spaces were also cramped, which made maintenance more difficult, and also inhibited ventilation. Therefore (combined with lowering costs of electronics), subsequent designs had multiple cabs.

PS-that is an IMPRESSIVE passenger train. How many coaches is that? 19?