Mr. Benton and other interested parties: some notes on the physics.
Aerodynamic drag is proportional to the square of the velocity; thus, doubling the speed quadruples the drag. Power required to overcome that drag is equal to the force times the velocity; thus, power is proportional to the cube of the velocity. Doubling speed, then, requires an eightfold increase in power.
125 mph is about 14% greater than 110mph; this is equal to an increase of about 50% in aerodynamic drag.
A typical locomotive is about 3 meters wide and 5 tall. With a midrange drag coefficient of 0.75 (about what I'd expect for a moderately streamlined locomotive), it will require about 1140 horsepower at 110mph and 1680 horsepower at 125mph to overcome drag. (This number is 630hp at 90mph and 420hp at 79mph).
A typical consist of locomotive and 6 passenger cars weighs around a million pounds. Rolling resistance is directly proportional to velocity; doubling the speed doubles the power required. Using a typical coefficient of rolling friction of .002, our theoretical train will require 590 hp at 110mph and 670hp at 125mph.
These results lead to several conclusions. First, that 125mph is significantly more difficult for a diesel locomotive than 110mph; in this example, it's about 60% more power required. Second, that if one locomotive can do 110mph then two can (if geared properly) do 125mph, since you're getting another 4000 hp from that second locomotive for about 200hp worth of extra resistance.
Third, we see how important streamlining is at higher speeds. Below 90mph, rolling resistance is the dominant resistive force no matter what your drag coefficient - as any commuter agency can tell you, you can go 90 driving a box twice as old as I am. With a typical diesel like the Genesis, by 110mph air resistance is the majority of your power consideration, and is completely dominant by 125mph. A well-streamlined trainset with a drag coefficient of 0.3 would let you go 150mph with the same power as 125mph requires with a standard train.
Fourth, we see why electrics dominate everything above 125mph worldwide. At 200mph, even that streamlined trainset requires 3800 hp in this idealized simulation, and 5000hp at 220mph. (A good estimate seems to be to double the numbers given to give actual power consumption rates). A Genesis gives 4250 horsepower, while even an old toaster gives 5800 hp. The ACS64 is speced for 8600 hp, while the Acela runs at 12,300hp with both power cars online.
