If the number of passengers is unknown, we can't guarantee it is strictly positive or even an integer.

Sure, accidentally killing lim {-2,-1,-2,-1...} people is good compared to lim {4,2,1,...} or 5, but only if it's an integer that's a 50/50 on being negative. If it's not an integer, which is infinitely more likely if we truly have nothing to go off and have to assume it was randomly chosen, then reality might break upon reaching the station.