Stockholm's metro system is a huge art gallery that started in 1957. It wasn't originally built with that in mind though, but over time, existing stations that were more bare-bones had an artistic retrofit. The metro system is now an attraction in its own right, and it is worth a few hours of your time to explore.
The platforms of T-Centralen, depicted in the first picture, bear striking semblance to the NYC Subway -- but the Stockholm metro system is of course much better maintained.
