O-GaugeCJ - Prototypical railroads interchanged/interchange freight cars on a daily basis. It was/is a common practice/ business as usual. For instance, since you mentioned New York Central, it would be common to see a NYC freight with cars from western roads such as Union Pacific, Santa Fe, Southern Pacific, and southern roads such as RF&P, Southern , Florida East Coast, plus midwestern and northern roads such as Boston and Maine, Milwaukee Road, Illinois Central, Rock Island, Chicago Burlington & Quincy.
Up until the 1960's before there were many mergers, well over 100 class one railroads existed in the US. All interchanged freight cars.
Before May 1, 1971 the birth of Amtrak, most major railroads ran their own passenger trains. There were nondescript ( ordinary ) type of passenger trains and there were name first class trains such as; the Santa Fe Super Chief, Illinois Central Panama Limited, B&O Royal Blue and National Limited, NYC 20th Century Limited, Pennsylvania Broadway Limited, Burlington Zephers, etc. It was not unusual to see some cars ( usually sleepers ) of a western road on a train of an eastern road. One of B&Os trains from Chicago to Washington DC included a Santa Fe sleeping car and one of Santa Fe's trains from Chicago to LA included a B&O sleeper. Some N&W passenger trains included cars of the NYC and RF&P.
