According to Leonard Maltin's Movie Guide (and quite a few other film reviewers) this is "one of the best B's ever made". I'd suggest that is a decent-sized overstatement. The plot has a clever twist and the script offers some moments of snappy dialogue but in the main it's efficient but familiar genre film-making from a time when the film noir was on its last legs.
Marie Windsor has one of her best screen roles as a tough-talkin' mobster's moll playing opposite Charles McGraw as tough-talkin' cop charged with escorting her via train from Chicago to Los Angeles in order for her to testify at a Grand Jury investigation into police corruption. The film opens with a nice switcheroo as we are initially led to believe that what are cops are gangsters but too much of the film involves scuttling back and forth between train carriages to be able to generate much enthusiasm (Fleischer's use of a hand-held camera to compensate for the limited shooting space was one of the earliest examples of the technique), only coming to life when Windsor spits out her acerbic lines
Shot in 1950 but not released until 1952 it was RKO's biggest hit for the year.
FYI: The film was re-made in 1990 as Narrow Margin with Gene Hackman in the Charles McGraw role. Director Flesicher's dad, Max, was the creator of Popeye and Betty Boop.