The train example is very informative because if you don't price the fine high enough you encourage people not to buy tickets. There actually was a mob-run "insurance" plan for train riders in India. The fine was about 20x the ticket price, but you would only be asked to show your ticket about every 100-200 rides. So you could buy half priced tickets from the mob and then they would reimburse you for any fines you paid while riding with their ticket.