Forty-and-eights were French 4-wheel boxcars used as military transport cars (the term itself refers to the cars' carrying capacity, said to be 40 men or eight horses). Built starting in the 1870s as regular freight boxcars, they were originally used in military service by the French army in both World Wars, and then later used by the German occupation in World War II and finally by the Allied liberators.
During the war years the cars ferried troops, prisoners of war, horses, freight, and infamously the Jews and others the Nazis considered 'undesirables' rounded up in France and sent to concentration camps and likely death in the Holocaust. Trains of 'Forty-and-eights' were frequent 'targets of opportunity' for Allied fighter-bombers operating over occupied France, since they likely held German troops or supplies; unfortunately sometimes a train of prisoners was indistinguishable from a troop train.
In 1949, France sent 49 of those boxcars to the United States (one for each State then in existence and one for Washington DC and Hawaii to share) laden with various treasures, as a gift for the liberation of France. This train was called the Merci Train, and was sent in response to trains full (over 700 boxcars) of supplies known as the American Friendship Train sent by the American people to France in 1947.