I'm curious about the fuel costs. Ignoring all other costs--and just looking at fuel cost per passenger mile--how does the cost of operating a train OMA-DEN compare with operating buses, assuming the train is full to capacity so you'd need an equivalent number of bus seats? Basically, I have no idea of the gas mileage of a tourbus, the gas mileage of two P42s, what type of fuel P42s run on (how does it compare to 87 octane unleaded?), and the number of seats on a tourbus (and hence the number of tourbuses required to bus a full train)?
Or in practice does Amtrak need far fewer bus-seats than that for OMA-DEN, since there are no intermediate stops (right? or are there?) and hence fewer passengers overall?
I'd guess Amtrak trains run on a somewhat lower grade of diesel fuel than a car does. A lot of newer cars require low-sulfer diesel fuel, while, for instance, while mine runs just fine on it, it doesn't need it. Actually, when its warm, my car can run on a huge variety of fuels more modern ones can't. My car runs ok on most vegetable oils, home heating oil (I've used it when I'm at my uncles ranch), peanut oil, waste vegetable oil, and so on. (No, I don't have a conversion to allow my car to run Straight Vegetable Oil from start up to shut down)
Most new cars can't handle any of the above. It would screw with their emissions equipment, their piezo injectors, common rail injectors, pre burn set ups, and so on. Also, it wrecks havoc on electric fuel delivery systems (mines purely mechanical). While I can't be sure, I can't think of a reason why the P42 would be equipped with such an engine. Such systems are designed for reducing emissions, and improving top-end horsepower.
Locomotives need low-end torque, not high end power.