The Reading, like many other railroads, published a "Summary of Equipment" document on an annual basis, listing all motive power and rolling equipment owned by the railroad, for both freight and passenger service. From these documents, one can ascertain the mix of different types of equipment - for example, the mix of 40' vs 50' boxcars, etc. They're also useful for developing your locomotive fleet on a proportional basis to get the right mix of manufacturers, road vs yard units, etc.