That's fair, but even interesting graffiti still won't let any sunshine come through. Also, I would rather look out the window [0] than appreciate artwork when riding on a train.
Considering the look of the train to the vast majority of people outside of it, I'm fine with not seeing anything - I'm staring at my book anyway for the most part, and there's another window on the other side. And I prefer it a lot over those ads that anyway otherwise contaminate the window with some random, probably sexist, racist, or otherwise shite nonsense.
[0] https://www.swr.de/swraktuell/rheinland-pfalz/trier/17134459...