Infantry is blue, not buff.
There undeniably were many open-top stock cars used in the 1860s. There is no reason to think that those cars were all scrapped upon Lee's surrender; there was too much expansion in progress for any rolling stock to be wasted. There is no reason to believe their use did not persist into the '70s and probably even the '80s. There is no reason to assume that they made it into the Far West--or to assume that they did not--but they certainly were used in the West (Alleghenies to the Mississippi).