As is typically the case with his noir reviews, JimBell's take on The Narrow Margin is consistent with mine, but more eloquent. This DVD is part of the excellent Warner Bros. Film Noir box sets (vol 2) that I gush about regularly.
This is not throwaway film noir material. In fact, it's one of those decades-old films that will make you say "why can't they make movies like that more often today?" A small but talented cast combined with mounting tension can make any plot enjoyable. Like 2003's Identity, for example. My only complaint is that Detective Brown didn't discover "Mrs. Neal's" body after the mob hood shot her in the back. Man, that would have been killer payoff, especially if right after that Brown chases the hood who jumps off the back of the train to hook up with mob reinforcements. It would take incredible self-control not to shoot the guy in the back, but Brown knew the Highway Patrol was en route. But... not everyone is as smart as me (including Richard Fleischer).