The 48 was a type of box car used in France during World War I. Originally called a “40 & 8” — after the sign painted on the side of each car indicating its capacity of 40 men or 8 mules — the 48 carried American troops to and from the front in World War I.
After the war, France gave the U.S. a gift of these boxcars, one for each of the then 48 states. *
“High 48” was railroad slang for the box cars on Hot Shot freight trains (fast trains that traveled non-stop from one large U.S. city to another), which were the conveyance of choice for time-conscious freight-train hoppers.
* Special thanks to Warren L. Bidwell for providing this information. Warren is a Korean War vet and a member of the Sauk County (Wisconsin) chapter of the 40 & 8, a veterans organization founded by American Legion members after World War I.