From their web site:
La Societe des Quarante Hommes at Huit Chevaux is an independent, by invitation, honor organization of male and female U. S. veterans, more commonly known as the Forty & Eight.
The titles and symbols of the Forty & Eight reflect its First World War origins. Americans were transported to the battle front on French trains within boxcars stenciled with a “40/8”, denoting its capacity to hold either forty men or eight horses. This uncomfortable mode of transportation was familiar to all who fought in the trenches; a common small misery among American soldiers who thereafter found “40/8” a lighthearted symbol of the deeper service, sacrifice and unspoken horrors of war that bind all who have borne the battle.
The Forty & Eight was founded in 1920 by American veterans returning from France. Originally an arm of The American Legion, the Forty & Eight became an independent and separately incorporated veteran's organization in 1960. Membership is by invitation of honorably discharged veterans and honorably serving members of the United States Armed Forces.
After the war, France sent a boxcar to each of the United States to honor the Americans that fought for France. To see the Oklahoma boxcar, see Waymark WMKDEH, posted by my friend Hamquilter.