Train from Paris to Chamonix

Sam, Chamonix-Mt. Blanc is in France although it is one of 3-4 cities outside of Switzerland that will sell you Swiss passes and cards. Most day runs from Paris to Chamonix-Mt. Blanc take 5.0-6.0hrs. However, as you have noted, there is a night couchette train that departs Paris-Austerlitz at 21:40 and arrives Chamonix-Mt. Blanc at 09:47. It's a 12hr run. You can see and buy this run on ww.tgv-europe.com. This is a French train website that will display in English. On the upper right hand side of the webpage, Great Britain as your "Ticket Collection Country". This will keep you from being switched to RailEurope is you U.S.A. Don't worry. You will be able to print your tickets out at home. You won't need to go to GB to get them. If you hurry, I see a special fare of 81.40Euro for August 12. Here's a LINK to what I see.

Here's a little more information to add to what Larry wrote. On the tgv-europe site the default choice is "Direct trains only." Uncheck that box on the home page or you won't see the route you need which requires a connection in St-Gervais-les-Bains. Even if you choose Great Britain as your country of residence, the site will try to direct you to Rail Europe (in this case the UK RE site). Stay with tgv-europe. Choose France as your ticket collection country. Since your second leg (St-Gervais to Chamonix) is on a regional train for which no reservations are possible, you won't be able to print your own tickets. Station ticket machines only accept chip & PIN cards so choose to pick up your tickets at the station ticket window. The special €81.40 fare includes a bunk in a 6-person couchette. If you want a bunk in a 4-person couchette, you'll have to book a 1st class ticket (€136.30). All couchette compartments are the same size. The only difference is in how many bunks are pulled down. You'll feel less "packed in" in a 4-person couchette.