Someone is paying to post philosophical statements in the subway cars. Could it be the STM on its own initiative? Deciding to educate the travelling public?
Here's one I like:
La croyance forte ne prouve que sa force, et non la vérité de ce qu'on croit.
It's from Nietzsche's Menschliches, Allzumenschliches (Human, All Too Human). In English:
Strong belief only proves the strength of one's belief, not the truth of what one believes.
I don't generally refer to Nietzsche for ideas, but I like this one.