I guess Charleroi (Belgium) might have a longer one. https://www.urbex.nl/metro-leger-charleroi-ligne-chatalet/ https://www.urbex.nl/metro-leger-charleroi-lign

I guess Charleroi (Belgium) might have a longer one. https://www.urbex.nl/metro-leger-charleroi-ligne-chatalet/ https://www.urbex.nl/metro-leger-charleroi-ligne-chatalet/ However, it's only 1 line that's unused, some other lines are in use.