Funny that you should mention that because there used to be Northam station near the maintenance sheds, but after the sharp right turn, serving the route from/to the Terminus station at South Western House, and thence across the road in the docks to meet the ships. I'm not sure whether that St. Mary's halt is what's left of it or whether it was nearer the maintenance sheds. Railway distances are always longer than my memory of them and I think it might take a true Southampton railway enthusiast to answer your question properly.
A picture paints a thousand words [now don't go on about the relative profundity of a photo]. Look here. I am about 90% sure this view is looking north, with the steel road bridge going over the tracks that turn for the tunnel. So Northam station WAS after the turn and the St Mary's Halt may well be a remnant of the old Northam platform. Which means that standing on the platform immediately in the foreground of this photo and looking across the tracks [looking right from this viewpoint] one would either be looking at the gasometers or directly at the spot where St Mary's now sits. Never seen this photo before, and I've looked hard in the past. please log in to view this image
Part of an OS map from 1910. It would seem that trains only passed through Northam Station so long as the old Terminus Station was in use.
That’s the exact route I use. For one season I went over the crossing and parked in that industrial estate where the tv studio was, but then found getting back over the crossing a pain, so parked on Empress Road. It’s a great spot for getting out of the city fairly easily. Not the most salubrious part of the city, though.
Almost exactly 4 years since Koeman had his Achilles injury, just before the Vitesse Arnhem game. Let’s hope this season goes as well as that one did!