I live near SLC and go get to San Francisco is about 18 hours, and that's a straight shot. Coach costs about $120, each way, which is about the same price as a non-budget airline. There's only one train each day and it runs from midnight to about 6PM the next day.

By car it's about 11 hours and about 2 hours by airplane.

So it's:

• slower
• not cheaper, perhaps more expensive if you don't mind budget flights
• less flexible - one train/day in most cases

There are tons of places I just can't get to, like Las Vegas.

If I was retired or something when spending more time was totally fine, I'd consider taking the train. But as it stands, it's just not a practical option unless the train is the destination.