I'm not entirely sure.. my memories are fuzzy.
https://fr.wikipedia.org/wiki/Gratuit%C3%A9_des_transports_e...
Caltrain has the same "honor" system, but conductors often do proof-of-payment checks.
It works partly because the fines are relatively significant and levels of public trust and integrity are likely more universal, so there are few "Schwarzfahrers" (fare dodgers, literally "black travelers").