Thuin Lobbes Anderlues
While strictly speaking the line at Thuin is really a museum, something I try to stay away from on this site because its as far from a true public subway system as you can get, I decided to include Thuin because it's a live, running "chunk" of what once spread out all over Belgium - 4900 raw km worth of a traction enthusiast's wet dream. The 6 km line, which once connected Thuin with Charleroi as part of Line 91, closed in 1968. It now runs on Sundays only, connecting Thuin with Lobbes and then Anderlues, where you can catch a tram into Charleroi. There is, according to this site, sparse passenger service between Lobbes and Anderlues.
Photo thanks to Kees Pronk