We'd like to get a taste of train travel in the Alps during our driving tour next summer. We're considering the Golden Pass from Interlaken to Montreux, and the Mont Blanc Express from Martigny to Saint Gervais-les-Bain. Opinions on the pros and cons of each would be appreciated. Thank you.
If you want to get between Interlaken and Montreux, or Martigny and St Gervais-les-Bains, its a fine way to do it, especially in good weather.
The Interlaken to Montreux route (complete with changing trains as the track gauge changes) is scenic but more forest than mountain.
There are other routes which go more in the mountains but they tend to be more in eastern and southeastern Switzerland.
If you want to go to the far west of Switzerland the routes you have chosen will work.
Trains transport you from A to B (obvious statement), if you want to do a train trip as part of a driving holiday, you need to get back to "A" to pick up your car. The big famous train routes are all taking you through the mountains from one part of Switzerland to another (Glacier Express route, Golden Pass route, Bernina route), leaving you 100's of Km from your parked car.
But, there are many rail trips going up mountains, and down again, which is probably suitable for you.
If you are in the Interlaken area, stay 1 or more nights in Wengen. This involves leaving your car in Lauterbrunnen and taking a train up to car-free Wengen. Equally car-free is Mürren on the other side of the valley. From both there are trains and cable cars up to mountain summits.
See this map of the area (red=trains, black=cable cars): http://www.regionalpass-berneroberland.ch/assets/karte-und-partner/Regionalpass-Panoramakarte-2017.pdf
Another place you have to reach by train is Zermatt.