La Roche-sur-Foron and the lines to Annemasse and Annecy were the birth place of "industrial" alternating current 25KV 50Hz for the French railroad lines electrification in the early 1950's. It was concluded that is was a significant improvement relative to 1.5KV DC. Consequently most new electrifications after the "Savoie" experiments were done with 25KV 50Hz (the French high-speed lines have also benefited from this progress). This explains the presence of two very different voltages on French lines, and the need for dual voltage engines.
On the picture, we see, far away on the left, an early dual voltage engine of the CC 25000 series, made with the help of Swiss industries. It was mainly an alternating current engine with ability to work at low power with direct current. The more modern and then brand new BB 25176 (built in May 1976) was a full dual voltage engine. There were 3 quite similar series: tje 25100, the 25150 and the 25200. The main difference between them was mostly the gear ratio (and top speed). In their early years, the 25150 served a lot in the Alps on the Savoie lines where they were well adapted. The train on the picture is a Paris Saint-Gervais. In La Roche-sur-Foron, trains such as this one coming from Aix-les-Bains and going to Saint-Gervais had to turn around. When the engine was moved from one end of the train to the other, it was a good opportunity for me to take some pictures, as I was often travelling on this line.