[4] A police detective plays a deadly game of cat-and-mouse aboard a train with mob assassins out to stop a slain gangster's widow before she can testify before a grand jury.

Densel, however, has boarded the train during a stop at La Junta, Colorado, and waylays Jennings, freeing Kemp.

Meanwhile, Densel and Kemp search for the payoff list and discover the fake Neall in her compartment; the music from her record player gives her away.

Then Kemp discovers a badge and police identification, identifying her as Chicago policewoman Sarah Meggs, hidden within her record player.

Brown uses the reflection from the window of a train on the next track to see into Ann's compartment, and he shoots Densel through the door without endangering her.

The film was based on a story by Martin Goldsmith and Jack Leonard titled Target, whose rights were acquired by RKO in 1950.

[6] Scenes aboard the train were shot on a soundstage at RKO Studios, using rear projection for background landscapes.

[8] According to Richard Fleischer, RKO owner Howard Hughes was so taken with the film he considered reshooting most of it with Robert Mitchum and Jane Russell to release it as an A picture.

[10][11] Hughes did assign Fleischer to reshoot sections of the Mitchum–Russell film, His Kind of Woman, with the screenwriter of Margin, Earl Felton, providing uncredited rewrites for the latter picture.

In 1952, critic Howard Thompson of The New York Times gave high marks to the low-budget film:Using a small cast of comparative unknowns, headed by Charles McGraw, Marie Windsor and Jacqueline White, this inexpensive Stanley Rubin production for R.K.O.

Crisply performed and written and directed by Earl Felton and Richard Fleischer with tingling economy, this unpretentious offering should glue anyone to the edge of his seat and prove, once and for all, that a little can be made to count for a lot.

[12]Later, in 2005, film critic Dennis Schwartz said, "A breathtakingly suspenseful low-budget crime thriller that is flawlessly directed ...

[13] Film critic Blake Lucas makes the case that The Narrow Margin reflects the "noir view" of an unstable and deceiving moral reality.

Hackman's performance was praised, but the later version is generally considered a lesser work compared to the original movie.