US trains travel over the Great Plains and through the Rockies, while hauling huge loads. Thus they have had to be much larger and more powerful than British trains, which travel hundreds rather than thousands of miles. A typical US wheel arrangement would be 4-8-4; the famous articulated "Big Boy" is 4-8-8-4, while British express passenger engines never got bigger than 4-6-2. Britain also built the first trains, which were of course very small, such as Stephenson's 0-2-2 "Rocket". The track engineers of the time did not make much allowance for the fact that engines would grow and develop, so the maximum space around the train for it to clear obstructions such as bridges and tunnels, called technically the "loading gauge", is still much smaller for British trains. Within these constraints British locomotive engineers built great power into their engines. Also, lots of parts such as pipes were hidden away to give a much cleaner appearance to the British locos. Another difference is that British railway lines are usually fenced off, while this is impractical in the USA. This is why US engines must have a cowcatcher.
The distinctive US steam locomotive developed from a 4-4-0 design called the "American". This is the classic loco to be seen in cowboy films - wood-fuelled, a fat smokestack to catch sparks, two or three domes, two outside cylinders, a massive headlight and a big cowcatcher at the front. It is all-American - brash, expansive and liberated. Compare this engine to the "City of Truro". The GWR developed a unique look for its locos which remained nonetheless archetypal British - smart, prim and proper. Cylinders hidden away, with only the safety valve and top feed disfiguring the boiler, and a copper-capped chimney. Why these two locos are compared, is because of the City of Truro's claim to be the first to travel at 100mph in 1903. While this might be true for UK, an "American", no. 999 of the New York Central system, recorded a speed of 112.5 mph in 1893, a full decade earlier.
The record breakers:
Left - 999 from New York Central. This engine has since been substantially rebuilt.
Right - 3440 "City of Truro" from the GWR.
Left - Classic early American, Central Pacific's "Jupiter", which witnessed the Golden Spike in 1868.
Right - Another famous British 4-4-0, Deely's Midland "Compound" , no. 1000.