Like everybody else I have at times been caught in the crush at the bridge but there is no room for a station north of the bridge, and that would still be the other side of the bottleneck. There could be a platform squeezed in behind the bike sheds but that would require trains to drive up the spur and then reverse back onto the main lines and then possibly change direction again, none of it ideal. I am aware that trains regularly use the docks spur for car freight and that they often stack up there for the cleaning sheds.
The real answer is to improve the bridge and I did read recently that plans are underfoot to 'widen' the existing bridge route by installing a new bridge alongside the current one.
http://www.dailyecho.co.uk/news/14408397.Up_to___250m_to_help_solve_traffic_problems_in_Southampton/