Rey:
Interesting question with a complex answer or set of answers.
A good place to begin is with the history of the American Car and Foundry company. For specific types I suggest:
Kaminski, Edward S., Tank cars from American Car and Foundry
Thompson, Anthony W and Robert K. Church, Pacific Fruit Express.
Check with the various specific railroad’s historical societies, They frequently have information posted or available which will be very helpful. Larger railroads often had special classes of rolling stock built, this was the source of considerable innovation which later became standard.
Federal regulation spurred changes, the Safety Appliances Act of 1912 (? on the exact year) led to considerable change in car fittings. The war emergency rationalization of rolling stock designs in World War I was a great change maker.
Change from one construction practice to another was often slow. Center sill cars were first produced in 1895, yet truss-rod cars continued to be manufactured until the start of WWI, a generation later. I believe PFE continued to order wood side cars into the 1950s.
Railroads continued to run outdated equipment for decades. I personally saw an X29 box car in San DIego CA in the late 1960s.
I would say your best bet, if you want to be period correct, is to pick a specific railroad and a specific type of car…say box cars…and research what the road was running at the time. Freight Car Cyclopedia for various years are available on CD for just this purpose.
If you really want to find the answers you are going to have to like, or learn to like, doing research. You will get considerable help here. Several of the posters are very knowledgable on this exact subject. I’m sure they will chime in, sooner rather than later.
Side comment, both Westerfield and Sunshine are good about researching the background of their kit offerings and posting it with their