In the United States, particularly in the industrial northeast (where I am from) there were two 500-pound gorillas with respect to railroads; the Pennsylvania Railroad, and the New York Central System. The PRR was slightly bigger - in 1927 they rostered 266,624 freight cars and somewhere on the order of 5000 locomotives. The New York Central was second only to the PRR - the NYC had 220k or so freight cars, roughly the same number of locomotives (the NYC had a LOT more switchers. or "shifters" in PRR parlance, than the PRR) and carried about 80% of the freight ton-miles the PRR did... and both were several times larger than the nearest competitors.
Modelling the northeastern US in the 1920s means you're both a PRR and a NYC modeller, whether or not you wish to be. Think of them as the LMS and LNER of the US at the time; their cars were *everywhere* and dominant. These two roads alone might make up 50% of the cars in any given train, even if the railroad operating the train were part of neither system.
The PRR is lovely to model; they followed a very regimented and ordered path of improvements. When they found a car type that worked for them, they built them by the tens of thousands; literally. The 10 most common classes on the PRR accounted for more than 80% of the 1920s freight car fleet, with some individual types (GL/GLa hoppers, GR/GRa gondolas, and XL boxcars) numbering 30000 or more. Thus, one need only build a few model types to quite accurately represent the PRR. They ordered locomotives of a single type for most given wheel arrangements, and they ordered a LOT of them. In the 1920s, the PRR was run with a handful of types of engines, with hundreds of a given type.
The New York Central... did not do this.
Not only did the NYC buy cars in orders of 500 to 1000 at a time, most of the time, they went through near constant revisions and design changes. The NYC did not have a nice, user friendly car classification system as did the PRR. Cars were arranged by the lot number they were ordered under and these orders number in the hundreds. In addition, cars were spread across the sprawling New York Central system, which included both the New York Central itself (itself an amalgamation of the Lake Shore & Michigan Southern and the New York Central and Hudson River systems) as well as the Big Four, the Michigan Central, the Peoria and Eastern Illinois, the Canada Southern, the Pittsburgh and Lake Erie, and numerous other smaller divisions. Their engines underwent a bewildering array of updates, upgrades, sidegrades, modifications, and changes which make any attempt to comprehend them something of a hair pulling event.
I am attempting to build something of a compliation of the "signature" cars the NYC used that I might wish to model. They seem to have used a handful of types. Boxcars in the 1920s would be of a few major types; 36 foot double sheathed cars and 40 foot all steel USRA cars, as well as 40 foot steel framed automobile boxcars. Gondolas seem to be of two types; 40 foot steel ones with hopper bottoms, and 42 foot composite ones. Hopper cars were almost all of various twin hopper types between 30 and 31 feet long. But how many of each type? Who knows?
Why do we as model railroaders think about this stuff?
Who knows.