Yup, this is an access gate for the railway folk, presumably an easy route to this end of the tunnel. It's possible there's somewhere public to snap the tunnel from—there's a bridge further out of town toward Sea Mills, and there's a chance you could see the tunnel from there, but today's walk is already quite long enough, thank you.

From here the Clifton Extension Railway (from 1877) would have joined up with the line from Hotwells (completed twelve years earlier) at Sneyd Park Junction, a few hundred feet further out of town, and then proceeded to Sea Mills and beyond, terminating at Avonmouth. That bit of the line still runs; it's only the bit from Hotwells to here that was pulled up to make room for the Portway road, making Sneyd Park Junction no longer a junction.

Here a little fact I like: when the Bristol Port Railway ran out to Avonmouth from both Clifton Down station at the high far end of the tunnel near Whiteladies Road, and also from Hotwells, they offered a special splilt return ticket that let you walk down the hill from your home to Hotwells, take the train out to Avonmouth, then return to Clifton Down instead, so you could walk down the hill to home rather than walking back up. A lot of the dockworkers took this option, and it gladdens me to find that even these hardy folk were so put off by the hills of Bristol that they went for the lazy option!