Another underrated gem:

"SUDDENLY" (1954), dir. Lewis Allen.
This taut (75 minute) suspenser circulates around a well-thought-out plan by a psychotic thug posing as a Secret Service agent(Frank Sinatra) to assassinate the President when his train makes a brief stop in a small California town. He comes up against the small town sheriff (Sterling Hayden in one of his best roles), his ladyfriend, her father (ironically a retired Secret Service agent--the plot thickens!) and the ladyfriend's 10-year-old son. Despite some obligatory (for the era) sappy dialog, this film is surprisingly tense and harrowing. IMO, Sinatra gives his best performance as the thug--alternately calm, competent and convincing, and then vicious, violent and psycho. Well worth watching!