If the corrosion seems excessive for the time spent in the water, there are several possibilities:
1) A trichlor tab floater is being, or has been used, and the floater has been too near the rail.
2) Water chemistry is way off. Especially low pH and/or high FC relative to the Cyanuric acid level.
3) Stray electrical current causing the steel to lose electrons.
4) Some of the anchor/cup hardware is not good quality stainless and the rust is coming from the anchor, and not the rail.
5) The rail has been stored in a corrosive environment and exposed to acid and/or chlorine.
6) The rail has been scratched or otherwise damaged.
316 stainless steel is one of the better grades of stainless steel. So-called Super austenitic stainless steels containing 6% molybdenum are one of the best types of stainless steel for use in marine environments that contain significant amounts of chloride.
If you can post a picture, that would help.
Does a magnet stick to the rail?