The top rail or bottom rail? They're both structural. If its partially rotted you shouldn't have too much of a problem, but... If its rotted out, the car could collapse eventually. In Michigan, fiero's like to be eaten away, so what you should do is buy some of this, a few quarts worth
www.por15.com and put about 4 coats on the frame rails and engine compartment, Thats what I did, but the engine was pulled and everything. I was amazed when I bought my car though cause it was a Michigan car all of its life, 157,000 miles on the odometer, and there was only some rust in the trunk, a lil on the cradle, and such, not as bad as it usually is. But I believe it was stored in the winters under a car cover.
