A very simple noir. I honestly don't think you can get more core genre than The Narrow Margin. The setting is reduced to a single train, there's a lot of men in hats trying to be as spiffy as possible, while the damsel in distress has quite the foul mouth of her own. Add a plot twist and that's your classic film noir right there.
Agent Brown is tasked to get the wife of a notorious gangster to Los Angeles, as she's willing to testify against the Mafia. They take the train, but their journey isn't as inconspicuous as they'd hoped. Before long many people are on Brown's tail, trying to convince him that the woman is better dead than alive.
The performances are rather poor and the setting isn't really put to good use, but the pacing is decent and I do prefer this limited location noirs. There's not much time to get bored and once the little twist has been revealed, the film races towards its finale. It's all a bit plain, but there's some entertainment value here.