I don't know if these were official back when the photo was taken, but I've kind of assumed the one in Redland (which is more of a garden I think than an allotment) is merely tolerated, as were those at Montpellier, rather than officially allowed.
The Redland one always looked just like any other allotment for the years I lived in Bristol (I was there for 35 years up to 2006, but can't remember now when I first travelled on the line). It often looked very productive, and I'm pretty sure it will have been officially sanctioned. The Montpelier ones, I think, are more recent, though there is a small allotment site hidden away just above the railway tunnel there. I had allotments for many years on the vast Ashley Hill site that overlooks the main line and its junction with the Severn Beach line.