Sealer won't do much for the splits and cracks in the 2"x6" on top of the rail at this point. If the railing itself is still solid, you might get another year or two out of it by washing and sealing, but it won't "turn back the clock". The damage is done. You could possibly just replace the 2"x6". Hard to tell from the picture about the actual decking, but the step treads are getting a little "iffy". They don't appear to be as bad as the top cap on the rail though.....
