The Narrow Margin

The story is set on a train running from Chicago to LA. Among the passengers, the former wife of a mobster, Mrs. Frankie Neil (Marie Windsor), travels under heavy police protection. The cops assigned to guard her include Walter Brown (Charles McGraw), a hard-bitten sergeant who fends off the attentions of various mobsters on the train who want Neil dead. On her way to LA to testify against her former husband, Neil needs all the protection she can get as the story unfolds. (official distributor synopsis)