Great videos, but I'm curious about the first two scenes on the first video.

In those two scenes, especially the first one, you can hear distinct chuffing sounds. Given the speed, and the blur of the rods, it seems like it should be a constant roar. How is it that chugs are distinguishable? Dubbed sound? But the coach sounds seem reasonable?

Locomotives with Gresley valve gear must have the three pistons operating at precisely 120 degree intervals. (Different spacings could be accommodated by different lever proportions, if there were any advantage to a spacing other than 120-120-120.) In order for the inside crank to clear the leading coupled axle, the inside cylinder of a locomotive with Gresley valve gear is typically positioned higher than the outside cylinders and angled downward.[2] To maintain a smooth flow of torque, the crank angles are offset from equal 120 degree spacing to compensate for the angle of the inside cylinder (e.g. 120/113/127 degrees).

The resultant timing of the blast from steam exiting the cylinders still gives these three-cylinder locomotives a regular exhaust beat

... those of us who know Gresley three-cylinder locos intimately know better. That ever-so-slight change in spacing results in a perceptible syncopation or "off-beat" sound, most perceptible at slow, hard departure. This even has its own name in British rail jargon: "the Gresley beat." The name was used for a Transacord recording of LNER three-cylinder locos, and later still for a well-known modular layout of the approach to Kings Cross still making the rounds of shows today.

The result of that "offbeat" syncopation is that even at high speeds one can manage to distinguish the beat of a Gresley conjugated-valve steamer as it "roars" down the track. That clip with Bittern is sort of an extreme example, but.....