The London Underground seems reasonably clean to me, but I used to live there. It's closed overnight, but it's also cleaned during the day — you will see a cleaner on many trips you make in the evening, and trains are often (usually?) cleaned quickly when they reach the end of the line.

Stockholm Metro also has some wonderful design, although it's modern.

[1] https://www.google.dk/search?q=leslie+london+underground+til...