The Narrow Margin (1952)

In director Richard Fleischer's fast-moving noirish crime-drama (with the tagline: "A Fortune If They Seal Her Lips... A Bullet If They Fail!"), followed by director Peter Hyams' inferior remake Narrow Margin (1990) starring Gene Hackman and Anne Archer - it told about a number of mobster assassins who were targeting a widowed gun moll and grand jury witness, who was being transported on a confining, cross-country Golden West Limited train from Chicago to Los Angeles, with a claustrophobic, tense atmosphere - there were surprise character twists and secret identities:

• the plotline: the escort of widowed gun moll and grand jury witness Mrs. Frankie Neall (Marie Windsor) by incorruptible, hard-boiled Detective Sgt. Walter Brown (Charles McGraw); the witness had information - a purported list of payoffs - that she was going to divulge at an LA hearing
• in the opening scene - Det. Brown and Sergeant Gus Forbes (Don Beddoe) took a taxi from the train station to pick up the dislikeable female at her apartment hideout to take her to the train; they discussed what the woman Mrs. Frankie Neall might be like (Brown: "Sixty-cent special. Cheap, flashy. Strictly poison under the gravy"); as they were coming down the stairs into the apartment's foyer, Forbes was gunned down by mobster assassin Densel (Peter Virgo), identified by a fur-collared coat

Sgt. Gus Forbes (Don Beddoe) Just Before Being Murdered	Detective Sgt. Walter Brown (Charles McGraw)	Mrs. Frankie Neall (Marie Windsor)

• on the moving train, where Detective Brown was attempting to hide Mrs. Neall in an adjoining sleeper cabin/compartment, two other assassins were circling around trying to locate and kill his subpoened grand jury witness: Joseph Kemp (David Clarke) and Vincent Yost (Peter Brocco)

Mobster Assassins
Joseph Kemp (David Clarke)	Vincent Yost (Peter Brocco)

• the interactions between Detective Brown and ill-tempered Mrs. Neall, both of whom took an instant disliking to each other: (Brown: "You make me sick to my stomach." Neall: "Well, use your own sink. And let me know when the target practice starts!")
• the vicious fight scene in a cramped men's room between assassin Joseph Kemp and Detective Brown
• further complications when Detective Brown became acquainted with another woman on the train - a golden-haired, sweet-natured mother named Ann Sinclair (Jacqueline White) with a precocious, overly observant son Tommy (Gordon Gebert) in tow and an older nanny, Mrs. Troll (Queenie Leonard); Brown worried that the mobsters might mistake her for his witness and kill her; Brown's loyalties were also tested by the DA via the decoy Mrs. Neall and assassin Vincent Yost to see if he would accept bribes of $25-30K to give up his protected witness: "You have her, we want her, how much?...Point her out and turn the other way. You go through the motions of guarding her until the accident occurs." Brown responded: "I'm not interested...not at any price"; the detective was warned: "We'll get her whether you give her to us or not, so don't take too long. It would be a shame if you missed your opportunity"
• Brown was aided by rotund railroad detective/agent Sam Jennings (Paul Maxey) (originally thought to be one of the assassins), who handcuffed gangster Kemp and detained him after Brown beat him up; however, at a train stop in La Junta, Colorado, assassin Densel surreptitiously boarded the coach, traded places with Yost, freed Kemp, and knocked Jennings unconscious; the two mobsters discovered Mrs. Neall in Brown's attached cabin, and in a dramatic scene, as she reached for a gun in her purse, Densel shot her in the back
• the twist revelation that Mrs. Frankie Neall was actually a decoy -- a policewoman from Internal Affairs named Sarah Maggs - who was shockingly killed; Brown's love interest - Ann Sinclair - was revealed to be the real Mrs. Frankie Neill; Ann described how she had found her divorced mobster husband's payoff list, and already mailed it to the DA; Brown reacted vehemently: "I've been played for a sucker!"
• the climactic scene in which Ann Sinclair was seized and held hostage by a mob hitman Densel; Brown used the reflection of another train's window to gun down the hitman without compromising her safety; Kemp fled and departed from the back of the train, but a few minutes later was arrested as he was about to be picked up by a getaway vehicle that had been trailing the train for many miles; in the brief conclusion, Brown safely escorted Mrs. Sinclair off the train