Have you looked at the Swiss Rail website?
for the English page. Swiss Rail works for schedules regardless of where you are departing and arriving and even if journeys don't enter Switerland.
You can take the TGV to Lausanne without changing trains and then change to the train to Montreux.
Some TGVs go all the way to Montreux (or they used to) and some only go to Geneva or Lausanne. I always opt for the train with the fewest changes.
Use the site kileysmom gave you above to find the best schedule. The discounted PREMs or Piccolo fares go on sale about 90 days in advance, but they sell out quickly. So buy as early as you can!
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