I hear ya. I understand that a train has virtually eliminated the wheel to track friction element...at least per unit of weight hauled. And, for road vehicles, I understand that the friction of tires to road is proportional to weight, but I just can't see it being in the top 3 in terms of variables causing an SUVs highway mileage to suffer vs a "passenger car". Accelerating the extra mass on flat; accelerating mass against gravity and wind drag due to the larger vehicle surely are #1 #2 and #3..??...and probably (just guessing...but someone would have to really do some convincing for me to think otherwise) account for 80+% of the observed reduced mileage per gallon of fuel with a SUV vs passenger car.
I have an "over the road" trucker friend (interesting business by the way). He says he gets, on flat ground, 11-12 mpg full (50,000 lb load). He gets 12-14 empty. Now, that's only a ~15-25% difference....and of that, how much is NOT due to accelerating the heavy load (thus is caused by the increased road-tire friction)? I don't know...but it seems like most of it is due to the acceleration of mass. Tires matter....but if you're keeping them at a good pressure (I go 40-42 psi on trips), just can't imagine that's where the weight, by itself, gets ya. Maybe I'm off on that..... I'm open to evidence.