> Gas isn't the only cost, but depreciation wouldn't be included,

Depreciation is driven linearly by mileage. It is part of the
$0.45 or so per mile that the IRS permits you to deduct for
business travel, for instance.

> nor would all the service costs,

Maintenance is almost purely mileage-driven (as opposed to
repairs). It likewise factors into the IRS rate. I figure that
the oil-and-filter change costs about $0.01 per mile, for
instance. Other maintenance items happen at essentially fixed
intervals (15k, 30k miles), and are therefore amortizeable.

> nor insurance (unless perhaps
> these trips bump you into a higher mileage and rate for
> insurance, but that's unlikely).

Agreed here.

> Unless this trip is somewhere close to the tipping point for
> whether Chris will own a car or not, factoring in the complete
> cost of the car isn't a proper analysis.

Another point: Amtrak loses *big* when you add another
passenger or two to the trip; *all* of the automotive costs are
essentially fixed (modulo the need to use a bigger or an extra
vehilce. They therefore get amortized per passenger, while the
Amtrak cost increases linearly per passenger.