When I discuss mass transit with people who don't have a lot of experience with it, I realize they tend to simply see it as an alternative way to go from A to B. As in, if there were a commuter rail system built parallel with the highway, then I could either take the train along that path or take my automobile. And commuter rail systems are frequently a lot like that.
But a comprehensive mass transit system isn't simply about establishing parallel competing transit nodes, it's about having a transit system that allows for a completely different kind of urban space and a completely different way of life.
The extreme example which might help highlight this idea is Manhattan. It's pretty easy to see how Manhattan simply couldn't exist in anything like its present form without a comprehensive subway, bus, and commuter system. The island couldn't support anything like that kind of population and employment density without it. Picture it as it is with one car per person and all of the parking lots to go with it. Doesn't work.
But that's true at density levels much lower than Manhattan, which also can't be supported, at least with decent quality of life, without a comprehensive mass transit system. I was just in Barcelona, which has an absolutely crazy ever expanding amount of public transit. The city's roughly at the scale of central Washington D.C., covered mostly in 6-8 story buildings, generally apartments or offices on top of street level retail. Tall office towers and apartment blocks, to the extent that they exist, are on the outskirts. City population is about 1.5 million, and roughly double that for the whole region.
Expanding the transit system there is less about giving people a better way to get from A to B, it's about making it fast and easy to get around everywhere, reducing the number of cars and the need to make space for cars, and overall creating a nice urban space which can only exist if you have fewer cars.
Obligatory disclaimer: not everyone wants to live in the city! I know! But the problem with a lot of US development is that it combines the worst of both worlds. You have cities which don't have enough transit and have too much car-friendliness which reduces the quality of urban life. And you have suburbs which are dense enough to have some of the negative aspects of urban life, but which aren't built to take advantage of any of density's benefits.
