The Gamma People [1956 - UK] sci fi

Mike Wilson, an American reporter, and Howard Meade, an English photographer, are on a train enroute to Salzburg. Their passenger car accidentally disconnects and proceeds over an abandoned track-spur into an mysterious country, Gudavia, where they are jailed as spies. A scientist named Boronski (aka Dr. Macklin) controls the kingdom and is experimenting with the effect gamma rays have on humans. In addition to creating genius children, Boronski's work results in zombie-like creatures. Those creatures are under his control and he uses them to stay in power. He orders Wilson and Meade released, fearing unwelcome bad publicity, but Wilson smells a story and begins investigating while they await transportation to Salzburg. The two outsiders are aided by Paula Wendt, who works in the castle lab with Boronski because she wants to rescue her brother, Hugo, from Boronski's experiments. Events spiral out of control requiring Mike and Paula to enter Boronski's castle to rescue a local girl. Hugo discovers their presence and they are captured. Boronski plans to eliminate Mike, Paula, and the girl using gamma rays.