A railway post office (RPO) was a railroad car operated to sort mail en route to a destination. Beginning in 1862, RPOs sped mail around the country, improving logistics and decreasing delivery time, finally ceasing operation in 1977.
These covers by Joshua McGee feature a detail of a black and white photograph from 1925 (held by the National Postal Museum in Washington, D.C.) surrounded by a digital frame and drop shadow. The background is a line extraction of railroad tracks stretching into the distance.
For postage, I used one tan First Class U.S. Flag sheet single (Scott #4129) and one Railroad Mail Car single from the Transportation Coils series (Scott #2265). They are postmarked with a "Mail by Rail Station" pictorial postmark from Pewee Valley, Kentucky, 40056 dated October 22, 2016.