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History Channel is "famous" for that. This the same channel that had a program about the development of railroads in the USA, complete with archival footage of British steam locomotives in action.

Now, don't think I am dismissive of British motive power. It can be argued that Great Britain was where the steam locomotive was invented. And GB was home to Big Bertha, arguably the most powerful steam locomotive of its time on the east side of the Atlantic. Furthermore, our English friends also built the legendary Mallard, which to this day, holds the title of the world's fastest steam locomotive (126 MPH, which in my mind, would be a frightening speed in one of those contraptions).

That said, about all British and contemporary American steam locomotives had in common was a boiler, some wheels, rods, and smoke. Even my wife, who knows almost nothing about locomotives of any kind, noticed that in a couple of scenes in the History Channel program the locos didn't "look right." Her suspicions were subsequently confirmed when she saw footage of the Union Pacific's Challenger in action.

Now, if my wife could tell the difference, why couldn't the History Channel folks?

"Most powerful" might be the wrong descriptor for Big Bertha, bearing in mind that the GWR's Star class (a direct ancestor of the Castle class) was already well established in 1919. She was designed to maximise tractive effort at low speeds, not power, and lacked a superheater which was by then becoming standard for express locomotives. Indeed it is unlikely she could have used all of her theoretical steam-generating capacity in her assigned duties, not least because the fire would not have had time to work up to optimal temperature before the summit was reached.

There is a remarkable anecdote about Silver Link, the first of the LNER A4 class to which Mallard belonged. The project to build both the locomotive and its matching set of streamlined carriages was approved just five months before construction was completed on both. Then, on an inaugural run with journalists aboard, this train proceeded to set a new speed record of 112mph, maintaining 100mph or better for a full 25 miles. Then, this still-unique locomotive proceeded to run the flagship Silver Link service for several weeks without a spare being available - the second of the class still being under construction - without a single failure or delay attributable to the engine. Just try matching that sort of project delivery today!

The A4 class was definitely on the vanguard at the time. At 184 short tons, the class was relatively light on its feet, which helped at high speeds. However, as was typical of Pacific types, the A4 was not very strong on starting tractive force.

American versions of the Pacific had somewhat more weight on the drivers and in some cases, were used in freight (goods) service. The Santa Fe, in particular, liked Pacifics, as the 4-wheel pilot gave them excellent stability at speed. The most elegant (and complicated) of the Santa Fe Pacifics were the four-cylinder, balanced compounds built by Baldwin, one of which was key to the success of Death Valley Scotty's record run in 1905.

The balanced compounds, of course, predated the widespread adoption of superheat to American steam locomotives. However, Santa Fe ordered simplex locos with superheat, once the value of superheat had been proved. That was the beginning of the end for compound locos in the USA.

The Pacific type wasn't as popular east of the Mississippi River, and the roads that offered high speed passenger service preferred Hudson types (4-6-4) due to the larger firebox that the type carried. That firebox resulted in a free-steaming loco that could continuously run at 100+ MPH all day with no loss of steam pressure. The Milwaukee Road's Hiawatha train was hauled by Hudsons that had a designed maximum speed of 120 MPH. There is no unimpeachable record of them actually being run at that speed.