To test the optimum technology for electrification with 25,000 V and 50 Hz, the SNCF had four different types of electric locomotives built by different manufacturers. Testing took place on the line between Lille and Thionville, where heavy freight trains had to be moved over steep gradients. In addition to the BB 13000, CC 14000 and CC 14100, the BB 12000 was built as a four-axle mixed-traffic locomotive with ignitrons as current rectifiers and DC motors which were supplied with a wavy DC current. Their special type of traction motors ensured smooth starting without slipping and tolerated a huge amount of overloading for a limited time. On an incline of one percent, they could haul 1.756 tonnes and even 2,885 tonnes on 0.5 percent.

In the end, the BB 12000 was the most successful of these four types. After the initial order of five, 143 more have been ordered. Like their sisters, they were called “Fers à repasser” (“flatirons”) due to their central cab design. The CFL in Luxembourg ordered 20 identical locomotives and designated them series 3600. While the first five had a top speed of 140 km/h, this was reduced to 120 km/h for the others to increase the pulling power. Also the first five were later rebuilt to 120 km/h. Since more modern locomotives already arrived in the sixties, they were soon replaced from the most important services. The last locomotives of the SNCF were retired in 2000 and the CFL retired their last ones in 2005.