When the train crashes off the railway track, all the carriages explode in fireballs. Whether the locomotive was powered by electricity or diesel, the passenger carriages would have carried no fuel and would not have exploded.
Contrary to a claim made, the United States Air Force, at the time the film was made, did indeed operate helicopters. In particular, Bell was awarded a USAF contract in 1963 for the manufacture of UH-1F helicopters for missile site support duties. Depicted in the film is the 1970 UH-1N, which in practice was used for special operations.