People who talk about what population density is necessary to "justify" a rail system are wrong but they're wrong in the opposite way from how they think. Even in Japan which has more than twice the population density of China the rail system is not profitable. JR makes most of its profit by operating malls and collecting rent from vendors. If you blindly follow profit instead of considering the broader social benefits the result will always be putting your rail system into a death spiral of rationalization. Stop expecting public transport to turn a profit that's not what it exists for.
The point of transit is to move people and goods from one place to another. If you expect that to be a profit center, you're expecting a private tax on all commerce as if that were a good thing.
the same people who complain about public transport not turning a profit also expect their government to build and maintain roads for them to "freely" drive on, as if those were not horribly costly. but that's road infrastructure, it justifies itself, it doesn't have to make a profit, what will we do without it, they will clamour, not realising THE SAME APPLIES TO OTHER FORMS OF TRANSPORTATION
The thing is having public transit will cause economic growth, and save people on the whole a bunch of other costs, much the way housing and preventative healthcare does.
You WILL see a payout, but it will be reflected in the GDP or whatever on the whole, while quite potentially looking like a loss when you just look at the transit system itself.
Public transit is a service. Like mail. Like healthcare. Like education. The point isn't to directly profit, it's to support people and other systems. You sink some money into street lights and having paved streets because of all the benefits you get out of that, not because having a street light is directly profitable.
It will enable people to access better work, better housing options, better childcare. It will reduce the costs and time waste associated with over crowded streets and reduce the cost to maintain that infrastructure, reduce pollution. That will have health impacts on everyone. Everyone's physical and mental health will get that little bit better. It will reduce accidents and emergencies and leave more room for emergency services/vehicles.
It's about thousands of little butterfly effects that come from having a population of people that's increasingly enabled to be at their best and to have options open to them.
