narrow gauge from France to Switzerland though the mountains

The contemporary Swiss line trains at Le Chatelard (Google Earth)

the Swiss part of the journey, old rolling stock

The Roger Broders poster created for the Chemins de fer de Paris à Lyon et à la Méditerranée, or simply PLM, is one that we have on our office wall. The Saint-Gervais to Vallorcine railway - which runs through the resort town of Chamonix - is now an SNCF line, but still metre gauge, and 36.5 km long. It is electrified with a third rail at 850 Volts DC. Although the steepest gradient is 9%, it is adhesion only. The highest point is at an elevation of 1,365 m (4,478 ft) altitude. Just beyond the border in Swiss territory it connects at Le Châtelard with another metre gauge railway, the Martigny-Châtelard Railway, which continues until Martigny where it connects with the Simplon line of the Swiss Federal Railways. This line is 19 km long and in part uses the Strub rack system which allows gradients of 20%.  The steepest adhesion only section has a 7% gradient.  This line uses 830 Volts DC, nowadays from overhead.

The Trains du Mont Blanc website

The TMR website