When I commute by train I bring the "beater" CAAD4.
When I decide to commute all the way home (33 miles one-way) I bring the nice bike (CAAD8 Campy).
I commute by bike/train everyday, except I ride all the way home once per week.
I just got some SKS full fenders to custom fit on to the commuter CAAD4.
Anyway Paula, I feel for you on the backpack. When I ride the CAAD8 home I make a conscious effort to put more weight on my arms to prevent using my lower back to support my torso & backpack. The CAAD8 has a fairly aggressive saddle to bar drop.
I am actually going to bring a set of work clothes at the beginning of each week to keep in my office so that I can eliminate the bag on my ride home day(s).