MDNFan wrote:[...] the lightly loaded train used 200-250 Gallons of diesel. [...] Same run, bigger load, used ~150gallons. [...]
I would guess that it makes little difference how full or empty the train is. Consider an eight-car train with roughly a 250,000-lb locomotive and eight 85,000-lb coaches. That's around 930,000 lbs.
A full passenger load in eight cars would be around 1000 people. At 150 lbs each, that's 150,000 lbs of passengers. That adds only 16% to the total mass of the train. (Even if I have somewhat over-estimated the locomotive and car masses, you can see my general point that the passenger mass is relatively trivial compared to the train itself.)
Even if the work the locomotive must do is proportional to the train's mass (which probably is not precisely the case; for instance, the locomotive comsumes the same amount of fuel at idle during station stops with or without passengers), that suggests that a run which would use 100 gallons of fuel running empty would use less than 116 gallons of fuel with a full passenger load.
That may come as some comfort to those troubled by seeing midday trains sometimes operating with only two or three of its cars in service. The added fuel that is used may actually cost less than the labor to uncouple those cars after the morning rush and perform an air test, then recouple them before evening rush. Particularly if the lights and heating/cooling units are shut off in the coaches not being used, thus decreasing the HEP load, the empty cars probably don't result in much additional fuel consumption.