Looks like it's just blocked off to vehicles rather than pedestrians...Never noticed a gap at the south end , if I’d seen one I’d have gone on to the viaduct , must be 20 years since I’ve been on it.
Victoria Viaduct © David Robinson cc-by-sa/2.0
Upon opening in 1838 this was one of the largest bridges in Europe, carrying the Durham Junction Railway over the River Wear. As the railways developed, this route was for a few decades part of the main London - Newcastle line, before the construction of the section between Tursdale and Durham...
www.geograph.org.uk