The movie is set in 1945 but here are some facts from the ship. Brazil Maru, a 12,752-ton troop transport, was completed in 1939 as a civilian passenger liner. Requisitioned in 1941, she a served as a transport during the first months of the Pacific war. Brazil Maru was torpedoed and sunk by USS Greenling (SS-213) near Truk on 5 August 1942.
The film's opening prologue states: "In February 1945, planes from the 493rd Squadron of the U.S. Army Air Corps bombed and destroyed bridges on the River Kwai in Japanese occupied Thailand. This film is based on the true events which happened during that period."
This movie is not an official sequel to The Bridge on the River Kwai (1957). It does supposedly follow-on from historical events as depicted in that film but the characters in both films are basically different.