North American verses European rolling stock... - Trains Magazine

Most modern Euro stock is just as large as US cars and bogie trucked just like US cars today.

If your talking about older cars, probably many reasons IMHO. In reality theres never been any real reason why the Brits or the Euros held onto the 2 axle goods van for so long, they could have switched to larger double axle boxcars long ago, afterall their coaches were everybit as large as their US counterparts. Car sizes were fairly comperable until the early 20th century, then US frieght cars started growing in size while their Euro counterparts sizes stayed relatively the same

I suspect it has more to do with just being traditionally used to working with over such a long period of time they simply saw no reasons to change. In the early 19th century Euro and Brit goods vans started as basicly road wagons on flanged wheels. The van builders were used to building road wagons so they simply carried on the same traditional building technics and sizes. Same for very early passenger coachs, as time went on even though the vans got a little larger and more durable, the same traditional size and type of vans held firm right into the 20th century. Distances traveled were much shorter than the US, loads and quantities involved were likely also lesser overall, so using smaller vans, say equivelent to delivery trucks here as opposed to semi trailers makes a certain amount of sense. Plus all people involved were used to the smaller vans, long time honored traditions die hard.

In the US, they found the more you could carry in a single car, the less it cost to ship over the greater distances in this country and was more effiecient overall, so market forces drove the car sizes larger. Plus as railroads evolved, tracks improved, ROWs were adjusted to be more efficient and faster allowing longer and faster trains, the smaller cars got unstable at high speed, so theres alot more incentive to build larger more stable cars for these faster freight trains, so as locomotives got larger and faster, so would the cars. 19th cent 20' cars gave way to 40' by WWII, then 60' and larger

This is all just speculation on my part, but theirs a certain logic to it.