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Been up to both a couple of times. Camp Muir is a little bunk house, also has an emergency radio for contacting the rangers. I slept in it one January. It works, but I prefer a tent. Long slow hike up and the altitude gets some people. If you don't do much climbing/mountaineering I'd recommend trying for Muir before you book a summit attempt.
Yes, and yes.Be forewarned: Rainier is a long slog of a mountain, both going up and especially coming down. The vertical climb is what you expect, but the horizontal walk is true drudgery--especially the downclimb in the warm summer sunshine, as the snow rots (melts), and each step will sink you 12 to 18 inches in slop.The climb to Camp Muir is not hard. The summit attempt above that is also relatively easy, but not to be attempted without either a guide, an experienced partner, or lots of experience on the part of the climber.You will find valuable information on Rainier (and all of the other major and minor climbs in Washington) at www.cascadeclimbers.com If you are serious about climbing the peaks of this state, register at that site and make some friends.
In my limited experience in this field I've learned you've got to take rotting snow into consideration. As in start early so you're walking up on frozen snow. Walk down ASAP to reduce the melting snow factor.
St Helen's is nothing compared to Rainier where proper training, knowledge, and technique are needed. Getting to Camp Muir is a good first step, but beyond there you need experience, knowledge, equipment, and the know how to use the equipment should someone take a fall. Already this year they are placing bridges across crevasses on the DC route. Here is one of the best sources for info and current conditions. Whatever you do don't just think you are going to climb it,mt he rangers will laugh at you and turn you around.http://www.mountrainierclimbing.blogspot.com/
And the DC route is the "easy" route too if I'm not mistaken.