In reply to Tonto:
Timetabling is easy enough here:
http://www.tgv-europe.com/en/ - just don't switch to raileurope.co.uk which is an agent, tgv-europe is just the English version of the SNCF website.
It looks like you can do Moutiers via Chambery from Geneva main station without having to faff around getting to the tiny SNCF station. I've done it from Chamonix which was easy enough but it looks like you get onto the Geneva train at Bellegarde anyway - so I guess it depends how much easier it is to get to Martigny than Geneva!