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I've been on Rainier twice, once in zip-off hiking pants up to the summit (with long underoos), and another time with softshell pants, long underoos, and full-zip shells.
Weather will dictate what you wear. Softshell pants are nice but not always needed. Fullzip shells are necessary (but hopefully not needed).
You'll be fine with a pair of long underoos (take your pick of midweigh options), a zip-off pant (particularly for your hiking/training), and a full-zip shell.
Down pants are not needed or recommended.
Last edited by pvnisher
on Thu Apr 04, 2013 12:33 am, edited 1 time in total.