Roadblock (1951)

http://www.imdb.com/title/tt0043973/?ref_=fn_al_tt_1

**** out of 5


Well, the title may be a bit of a spoiler for the ending, but it's also a good metaphor for the story, as well, which is part of what makes this an excellent example of film noir.  The tough guy Charles McGraw is good at his job as an insurance detective, but it's a job with little future, at least not in the eyes of the girl he falls for.  So he uses the information he gets in that job to plot a big train robbery.  Naturally, this plan unravels for him, as you would expect in any proper film noir, and he quickly finds himself backed into a tighter and tighter corner until he has no escape from the consequences of his fateful decision to turn to the dark side.  Joan Dixon is the girl of his affection, Louis Jean Heydt the partner whose sharp investigative ability and doggedly righteous character he ends up fleeing, and Lowell Gilmore the mob boss he ties up with to attempt his big heist.
There is genuine tragedy in this story, when Joan Dixon has a change of heart, and marries McGraw, only to find he is no longer the honest man she rejected before growing to love.  Her sorrow is the most moving when McGraw meets his final end, as she observes that demise, knowing it was her own greed that drove McGraw to that end.