Stained Glass Metro
One of Lyon’s metro stations, Place Guichard, has a stained glass wall. It is only really visible from inside the station, behind the ticket barriers. Just like the windows in a cathedral, it is dark from the outside. You need to see it from inside where the sunlight can illuminate it from behind.
I pass through Place Guichard regularly, but rarely have need to get off there. When I absent-mindedly got on the metro heading in the wrong direction the other day, it was a great opportunity to take some photos while I waited for the right train.