Given that there's hardly anything interesting about rocks and the occasional flash of light in a metro tunnel, I think there's a lot that can be done to make the metro experience better. Here's one idea I have to solve this international crisis which is oh-so-more-important than anything else on this planet. I call it, metro cartoons (the animated ones).
The basic idea is to insert cartoon panels between the tunnel walls and the tracks. Since we know the approximate speed at which the trains move, you can estimate the minimum spacing between the panels that would be required to provide a reasonable frame rate required for the animation. I'm sure artists understand these aspects better (thus, I don't), and can come up with the right kind of drawings for this setting.
What say? I'm pretty sure someone must have tried this already, so let me know if you've heard of any such attempt.