More riders = more revenue
That is true.
OTOH, more tracks, more trains, more stations, more conductors, engineers etc. = more expenses.
If revenue > expense, that is good.
If not, a point will be reached where it is time to cut bait.
Unless, of course, the government gets involved, which means that money that would go for things people want (as per choices made with money they earn) will got to things people don't want (as per money taken from what the earn via taxes)