Powderkeg (1971)

"Their gun and guts for hire... and a blank check is their price"

A Mexican bandit is about to be executed in the United States, so his brother takes over a train and holds the passengers as hostages unless his brother is released. Now both the Americans and Mexicans are baffled as to what to do. One of the passengers — who wrote the letter for their captor — has a suggestion: call mercenaries Hank Brackett and Johnny Reech. They do, and as expected they do come up with a plan, but the president of the railroad is not sure if it will work.