I’m sure enough air is moved simply from the train moving by, but there will probably still be rocks and stuff flying around

won’t really help against bird poop and such

i’d be more worried about about smudgy stuff. they get dusty, then it rains and the panel is covered in a film of dirt. bird shit on solar panels is already plaguing home users

Trains with hoppers are not present on all railways though.

This is Switzerland, outside of a small number of corridors the majority of tracks see virtually nothing but passenger trains.