Whenever I have tried to get a ticket it seems like they are booked months in advance. So far as I can tell, Amtrak is for people who are afraid to fly and who don't want to ride the bus with people who ride the bus.
As an example: Taking the train from ~OC/San Diego to Santa Barbara is absolutely beautiful, way better than staring at bumpers for ~3 hours trying to make it from orange county to the other side of LA. You can do it via flight but the cost doesn't make sense to fly into SB.
If you were going to NYC from CT (Metro North) via car you pay a ton in tolls then need to spend $15-$30 a day for parking AND traffic.