Multiple questions, multiple answers, maybe:
FL-9s in Jamaica, Queens, on display? It's possible. The FL-9 concept had potential users with the LIRR, NYC, LV and PRR/NY&LB. None panned out, although, as noted, they did end up on LIRR & NYC eventually.
Freight car per diem for off-line usage? Already mentioned, and there were tariffs published of a similar nature for passenger cars operating off-line (NH cars in Florida). Head-end cars, most notably "baggage" cars, could and did end up all across the country. The so-called "baggage" cars were also concurrently Railroad Express Agency cars, and when in REA service went wherever they were needed or loaded to. It was not uncommon to see the AT&SF's Fast Mail with NH (and other) "baggage" cars. Likewise, it was not uncommon to see ACL cars on the New Haven (along with the usual PRR B-60s, R-50s or X-29s) in REA service. PRR head-end cars could, and did, end up in Grand Central Terminal, just as NYC head-end cars could, and did, end up in Pennsylvania Station.
NH cabooses in express service routinely ran between Boston and Washington, or Boston and Pittsburgh, with PRR cabin cars running all the way to Boston. Payment to Railroad A for using Railroad B's rolling stock was either paid through per diem or balancing out accumulated mileage.
Speaking of which, when a dedicated freight routing was established between multiple railroads (e.g. four carloads of widgets each week from Waterbury, Connecticut to Spokane, Washington), with its agreed-upon freight rate, the railroads involved in the routing (NH, EL & MILW) would each contribute dedicated freight cars to cover the service based on each road's proportion of the total routing, mileage-wise.
Actual routing of freight cars depended on many variables - shipper-dictated, receiver-dictated, tonnage guarantees, tariff rates involved, time involved, etc. Routing ABCD could be the fastest, but most expensive, while ABED could take longer, but be cheaper. Sometimes time was the determining factor, sometimes speed was.
All of the preceding on rates and routings would take a book to discuss in detail, and even then it wouldn't be a complete discussion.
Ex-NYNH&H SS Opr