well then I would think you'd be wanting a big travel backpack, a ski bag and a hiking pack. Reasoning being I doubt you'll be lugging your entire trips worth of gear up and down glaciers so the ability to separate your gear is pretty key. Your skis will always be the same size so having them constantly strapped to you pack will make it extremely awkward on trains to maneuver around. A large pack on your back is quite easy, and having your skis in their own bag you can work those around either slung over your shoulder or find a way to strap them on your pack as needed. I have a 65 liter pack that has enough volume to hold a pair of ski boots and up to 2 weeks worth of gear, and skis can A-Frame up the sides if needed. A slightly larger pack would probably be a good way to go for longer trips