The design of subway stations generally sucks, with a few global exceptions. The Line 11 platform at the Arts et Metier metro stop, in Paris, is the most beautiful subway station I've ever been in. These are two shots I took of it in 2005.
The steampunk-style station had been redesigned that way in 1994 by Francois Schuiten, and though the photos may not convey it well, the copper lining the walls gives the space a warm glow you don't often find underground.
It's that warm unearthly light that makes Arts et Metier beautiful, at least to me. But I'd settle for earthly light, or anything besides fluorescent bulbs, to pretty up a station. The winning entry in a recent design competition in Tel Aviv, for instance, shows us what a subway could look like if lit by the sun.
Designed by architect/interior designers Galmidi Yitzhar and industrial designer Yaksein Eliran, the concept was inspired by the Ficus Microcarpa trees that have been planted in Tel Aviv to provide shade. Underground, Yitzhar and Eliran use the form of the tree conversely, to provide illumination through skylights.
[The design's intent is] to imitate and enhance the ambiance and feel of the outside world and to bring the street into the train station. That feeling is expressed by bringing in natural light as well as creating the image of the Ficus, i.e., a trunk with a bouquet of steel pipes that are placed on the train deck.
...The colours in the train station provide an additional interpretation of the multifaceted design and constitute a reminder of how 'white' and full of light are the streets of Tel Aviv. The whole station is thus painted white, a colour that not only symbolises clean lines, but also serves as a reflector (enhancing natural light that comes into the station) and as an additional source of light for the whole underground deck.
The concept is a mere single story below street level, making sunlight relatively easy to come by. But I could see this concept working for deeper spaces by using, for example, the remote skylights proposed by Dan Barasch and James Ramsey's LowLine.