In a word, it's time. If I want to drive somewhere in my town, it takes at most 15 minutes. A bus is easily 30-45. I think over long distances rail allows you to save time, or at least break even.
A train route was much slower than driving an ~80 mile stretch in my experience
Highly dependent on where you are in the country. Trains win in highly congested areas due to the dedicated right-of-way. With no congestion, cars almost always win.