context: Sometime in the mid-1960s, mercenary Bruce Curry (Rod Taylor) is hired by Congolese President Ubi (Calvin Lockhart) supposedly to rescue the European residents of an isolated town about to be attacked by rebel Simbas in the Congo. But in reality his mission is to retrieve diamonds, worth fifty million dollars, from a mine company's vault. Curry's subordinates include his friend Ruffo (Jim Brown) and alcoholic Doctor Wreid (Kenneth More). He also reluctantly recruits ex-Nazi Henlein (Peter Carsten) because he needs his military expertise and leadership skills.
  The Congolese president provides Curry with the use of a steam train and soldiers. On the way, the train is attacked by a United Nations peacekeeping plane. The mercenaries then pick up Claire (Yvette Mimieux) after they find her house burned down and husband murdered by Simbas. Meanwhile, Henlein, who resents Curry's leadership, begins to cause trouble because the German knows about the diamonds. Things come to a boiling point when he kills two children whom he suspects of being rebel spies. Afterward, Henlein makes romantic advances towards Claire, which Curry interrupts. Curry and Henlein then fight an inconclusive duel which involves a chainsaw. Curry is prepared to kill Henlein, but Ruffo stops him.
  Further complications arise when the mercenaries reach the town. First, the diamonds are in a time-locked vault delaying the train's departure. Second, Dr Wreid attends a pregnant woman at a mission hospital and refuses to abandon her. Curry reluctantly agrees to let the doctor stay behind. Back in town, Curry waits anxiously for the vault to open.
  The delay allows the Simbas to catch up and begin attacking the town and station. Finally, the train, loaded with the diamonds and fleeing residents, slowly leaves the station under small arms fire, but a mortar round destroys the coupling between the last two carriages. As the rest of the train picks up speed and steams away, the last coach with the diamonds and most of the Europeans on board slowly comes to a stop before rolling downhill back into the Simba-held town.
  Curry and Ruffo set out to retrieve the diamonds from the rebels at nightfall. Using a Simba disguise, Ruffo carries Curry's 'lifeless body' into the town's hotel. Its rooms contain graphic scenes (for the film's time period) of male rape, murder and torture. Following a diversion by government soldiers, they get the diamonds and escape in some vehicles. When they run low on fuel, Curry leaves to find some. Henlein uses his absence to kill Ruffo in the mistaken belief that he has the diamonds. Henlein flees into the bush. When Curry returns to find his friend dead, he is filled with murderous revenge. He pursues Henlein and kills him after a vicious fight. Curry then returns to the truck convoy. With his job done, Curry reflects before turning himself in for a court-martial to answer for his actions.
question: How do they travel to the diamond mine?
answer: Steam Train