Superman #54

Direct

Superman finds himself in Warsaw, Poland in June 1943 about to be hit by a Nazi train. Superman flies over the train, alerting some Nazi officers. He lands in the Warsaw ghetto where he notes that history has been changed. There, Superman discovers a plot involving General Zeiten regarding plutonium and atomic weapon experiments. Superman races to catch the train carrying the bomb. The Nazis, believing that the flying man could do more damage than all of Stalin's armies, decide to drop the bomb. Superman sees the bomb, catches it, and flies into space. He disappears in its massive explosion.