Yes while it is true that for 30x the investment you move much more then 30x the people, however this is quite an unfair assessment. At best you would need to compare Amtrak's trackage fees to the highway number and even that is apples to oranges. Of course I am telling most of the people here nothing new. Now if you add in to the highway number the total spent on local roads, what individuals spend on car costs, upkeep,fuel and insurance and throw in police and other emergency services you might come close.
Well not really, Amtrak has a couple of other issues that distort the numbers in favor of autos. For the most part if Amtrak were larger it would clearly have the potential of being more efficient. If it's fleet was in the several thousand range, the ratio of spares needed would be much smaller plus they would have more flexibility to add capacity for high demand special events.
Also there is a bit of a rules disparity. If autos had to be built to the crash standards standards that is forced on Amtrak by the FRA, 10 MPG would likely be very good. Amtrak is forced to buy overweight one off equipment from the rest of the world which if not for the efficiencies of steel on steel would make better immovable objects then a form of transit.
Now to give the car their due, there number is also higher then it needs to be. If highways were built to only handle cars, the maintenance and construction cost would be much lower. The reason why despite that big number our highways are in such terrible shape is because the interstate system is basically a subsidy for very heavy trucks. They may state proudly how much taxes they pay, unfortunately that number falls short of the actual damage they do to the roads and bridges.
Now I know I am preaching to the choir here but it's good to get it off my chest once in a while.