It's more like, with rail, you build the line, and if it's a useful line then the neighborhood around it will become popular. In NYC throughout the late 1800s and early 1900s, they built rail lines through farmland and the (medium/high density) neighborhoods followed. In LA, we're seeing better rail access revitalize areas like DTLA (or at least contribute to it). It's always a hard sell building rail lines through areas already connected by highways/roads, but if we watch LA over the next few decades we'll see what happens.
If you build a bus route into the middle of somewhere, even a good/reliable one, it probably wont change the neighborhood or stimulate nearly as much development. But hey, now you have a bus line. I used to ride one from my suburban high school in upstate NY into the small city's downtown, and it was fine.
I've always thought that was an advantage for buses. With rail, you have to plan it to go through areas that are not only popular now, but will be popular in 30 years. If some neighborhood suddenly becomes the happening place 10 years from now, you have to build a whole new, expensive line. But with buses, you can just add a new route.