arseny
rolling stock roamed freely all over the US network, and into canada and mexico too, i believe, as long as it complied with rules regarding interchange. so, for example, a boxcar with archbar trucks, after a certain date, might be restricted to the home railroad and could not be interchanged and used elsewhere. the railroads had a system of fees worked out to compensate each other when they used another road's wagons.
locomotives, well you'd often see track sharing agreements between two railroads or interchange in yards where you'd see the locos of two roads side by side. or they'd run a service together so for example, the premier freight train of the central railroad of new jersey (CNJ) would have erie lackawanna locomotives on it.
also railroads often leased power from related lines, or brokers who specialised in leasing. so for example you could see ex-richmond, fredricksburg & potomac F7s in their handsome blue and silver scheme, but owned by a leasing company, on the lehigh & hudson river up in new jersey.
sometimes railroads bought locos secondhand and were too poor to repaint them, eg rock island who bought GP7s from the D&RGW and new york central subsidiary, P&LE, and E and F units from the UP.
http://www.rrpicturearchives.net/showPi ... ?id=640359http://www.rrpicturearchives.net/showPi ... id=1389790http://www.rrpicturearchives.net/showPi ... ?id=466778when railroads merged, you'd often get a great variety of locos in different colours side by side. some railroads didn't bother to repaint equipment fast, or they didn't have the cash, so for example, the illinois central gulf continued to have gulf, mobile & ohio red diesels on trains about twenty years after IC and GM&O merged.
whether burlington and DSS&A locos ran together, i doubt it, but it is possible in theory. you'd need to check the history of the individual railroad.
a UP crane, burlington locos and D&RGW boxcars could probably be seen, in theory anyway, at afew places as these were large railroads that interchanged in a few places.
ben