It's also worth noting that rail first occurred in a context where cars simply did not exist. Of course it's going to be a dominant transportation mode if the only competition is feet or a smelly pack animal.

All-at-once joint property development/heavy rail construction still occurs today in many places. As in, there is a train station is built the middle of nowhere, and the area around the train station is developed. Some examples that are visible on Google Maps:

• Netherlands: Amsterdam and Rotterdam have metros that extend into their farthest, clearly greenfield, clearly post-WWII suburbs.

• Madrid, Spain: Subway stations in obviously new developments.

There is nothing inherently better about rails except ride quality. Technology fetishes like these are how we get toy streetcars instead of frequent bus networks.

Capacity and economies of scale. I mean, busses could run every minute or so, but they would have to give up their signal priority (or grade separate, which is massively expensive.), otherwise cross traffic would never get a chance to go.

Also, to increase capacity you have to hire more bus drivers (which is a massive chunk of a bus system's operating costs). Rail systems can just add more cars with the same amount of operators, saving a significant amount of money. Busses can't get any longer than bi-articulated.