The ‘subway’ used to link the different Torchwoods together (Besides Cardiff, there is the one in London that featured in Doomsday, one in Glasgow, and apparently one that went missing). The network fell into disuse, so now it’s where they have pizza and beer. As realistic as all these tiles are, when you touch them you can tell that they’re painted wood and paper.
The Welsh dragon on the wall is a nice touch. Apparently Tosh was painting it there, but that backstory never featured in an episode. I guess it never will now.