For cross-border traffic between France and Italy, the line over the Mont Cenis Pass had existed since 1871, the highest point of which was the Mont Cenis Tunnel, which was initially 12.2 km (7.6 miles) and later 13.7 km (8.5 miles) long. On the Italian side, the line was electrified between 1912 and 1920 with 3,600 volts three-phase current. At that time, on the French side, on the section also known as the Maurienne route, steam locomotives still had to struggle over ramps with a gradient of up to three percent. Therefore, the decision was soon made to electrify this route as well. Later, the lowland line from Marseille to Nice was to follow the same system.
Because it was easier to implement, a direct current system was chosen, and the voltage was set at 1,500 volts. Since this relatively low voltage and the required high power needed high currents, a large cross-section of the overhead line was necessary, which presented a technical and financial problem. For this reason, the decision was made to use a third rail next to the track to pick up the current. Since the third rail was difficult to implement in station areas with many switches, a conventional overhead line was installed there. Since there was usually no high power demand, the current levels remained within acceptable ranges.
In order to determine the best type of locomotive for this line, different prototypes were ordered from four different manufacturers in 1922 and delivered from 1925. All received pantographs on the roof as well as side contact shoes for the third rail. The four prototypes were followed by ten each of three types of freight locomotives and four express locomotives. These were only used on this line and had no competition until the fifties. It was not until 1960 that 22 additional examples of the equally old BB 1-80 of the former Paris-Orléans were added. Although these were weaker than the PLM locomotives, they were always used double-headed. When the more modern CC 7100 and CC 6500 came along, the career of the PLM locomotives quickly came to an end. Today the line is still operated with 1,500 volts direct current, but completely via an overhead line.