The common side window "sun shades" -- the proper term is cab awnings -- are spring loaded, not coil springs, but a single-leaf spring, so that they will stay either open or closed. By rule, they are supposed to be closed on trailing units. As far as rain protection, they are better than nothing, but some of them allow some rain to drip down between where the awningand the cab side are bolted together.
The older canvas type used on steam engines by some railroads, and also lasting on diesel applications on locomotives built up to 1960, were a heavy steel wire frame with canvas attached, and the canvas was also attached to the cab side using wood blocks. When extended and the locomotive was traveling at high speed, the canvas puffs up like a sail and rattled the steel frame endlessly.
