I made the same calculation getting from San Jose to Portland. I wanted to avoid air travel, so I checked the train. I'm taking four people.

    Air: $628 and about 5 hours (security, parking, etc)
    Train: $312 for coach, 21 hours (driving on each end)
    Train: $750 for a room for four
    My van: $120 gas, no hotel, 12 hours door to door w/ stops.  

The car is maybe $300 if you include wear and tear. But as a bonus, I have a car for my week in Portland. To have that same benefit with the other transit, I'd have to rent a car for a few hundred dollars at the other end.

Heck, I could pay someone to drive my van for me for about $150 each way, and still break even with the train but save time.