I believe strings of "mostly the same" tank cars has been an industry practice for a long time. I'm particularly fond of Sinclair tank cars and company history, and as vividly as 55 years of memory will allow, remember standing on the platform in Rock Springs, Wyoming in 1965 - 68, waiting for our summer camp group to arrive, seeing long strings of identical tankers blasting by on the outer tracks. The Sinclair refinery was +/- 150 miles to the east, at Sinclair, Wyo.
The really CRAZY part of that is that in trains going west, those long strings of tank cars passing the Rock Springs station in the afternoons were pulled by giant UP diesels with a hole in the middle!! You could see right through them!! It took 40 years for me to figure out those giant UP diesels, usually 3 in a row, were DD35's. DD40's didn't exist in '65, '66, or '67, but I might have seen them in 1968.
Anyway, whenever I've seen long strings of tank cars, present day or past, they are "mostly the same", maybe with different road markings, but generally the same construction type. If you're constantly shipping 100's of thousands of gallons of the same petroleum products to the same destination, why not use all of the same cars? And stored cars seem to be "mostly the same" on the tracks where I've seen them stored.
I've enjoyed this discussion immensely, and got a lot of info out of it.