[throwawayboise]

With trains you are limited because you need a certain amount of separation between trains for safety, and station platforms are only so long which limits the number of cars that can load/unload. And at the final terminal (typically the main downtown station) there are a finite number of platforms which limits the number of trains that can be in the station at any one time.

Agree that you can move many more people on one set of rails than one highway, but it's not unlimited.

[Gwypaas]

Regarding the terminus stations, in Europe many central stations are being converted, or where built as, through stations. This allows for much higher throughput since trains don't have to turn around.