For the railroads I have studied and read up in company files, I'll make a generalization that: specialized cars are most likely to be found in dedicated service.
The rational is that these costs of the (then) new designs had to be justified by a dependable ROI.
When the NP ordered new 50' cars in the late 1920's, these new 6000 series were to be dedicated to auto service. The studies done by them indicated that shipments of autos at that time were most dependent on what empty cars were available in the local Detroit area yards, and who let cars sit around idle local to the auto plants waiting for the call. They wanted these cars yesterday if not sooner, so they bought 1000 of these based largely on an existing AT&StF design that a car builder was building and had the assy. jigs already in place. As autos grew in length, the Evans loaders needed to be changed more or less yearly with each new auto development. Eventually for lack of several inches (3 - 4 inches) of interior roof height, the cars were no longer viable as auto carriers and were released into general service.
Later came covered hoppers, at a time when these were still uncommon on railroads, the NP bought a handful - for one shipper per the request of the sales dept. Then later bought the next handful for another shipper, again per the sales dept. Both of these sets and many later purchases of covered hoppers went directly into dedicated service. The SP&S and GN were the same early on in the development of covered hoppers from the exchange of letters found in corporate files. The Q had a bigger heavy industry market (the Chicago area) than the others and so was on an earlier timeline.
More NP examples (covered gons) exist however they are beyond this lists 1960 end date.
And I will state that if it was a dedicated service car, accounting had already crunched the numbers on the sales department request for said cars to be dedicated. An AFE or reading the railroads Board of Directors budget request for the prior year that cars were ordered for Oliver Farm Equipment or others should shed light on things.
Jim Dick - St. Paul, MN