Also, most British trains were very short, by US standards, and the lines had broad curves.
Part of the reason we have such big freight cars is because our early railroads had lousy track. Huh????
The British had a small country, with a large population, established industries and transportation systems, and a lot of money. They could afford the absolute best track, and that's what they built. The early freight wagons were just that - wagons, with flanged wheels. They worked fine, and by the time larger capacities were needed, the railroads had been built, the loading gauge established, and wagon size was limited.
The US had a large country, sparsely populated, little heavy industry, and limited transportation options. We need to haul freight long distances, through frequently undeveloped lands, at minimum cost. We built the cheapest track we could, to the lowest standards we could, just to get the trains running. We quickly found out that a traditional wagon wouldn't work.
So, we put two very small wagons under a larger platform. That spread the weight, allowing for smaller rails, and gave greater flexibility, allowing the "car" to stay on poorly laid track. Since the railroad was often the first into a new area, it didn't have to fit through established towns and farms, so could also have more generous clearances. With the newly developed "trucks" under a "car", the bearings weren't directly attached to the side sills, and the "car" could be wider.
As track, and bearings, improved, the cars grew, and today our freight trains are much larger than the typical British trains.
In passenger service, it went the other way. The British wanted quick, comfortable travel, between often closely spaced towns, while we simply wanted to get somewhere usually too far away for convenient horseback or stagecoach travel. Their railroads specialized in passenger service, and the freight had to fit as best it could. We specialized in hauling goods, and the passengers were basically another type of freight.
