This is a good film for anybody who likes Hitchcock, Film Noir, Detective or Romance movies. It manages to hit all these bases nicely. There is excellent cinematography, a reasonably involving script and some very surprising, yet natural, twists in the plot. While the tension does not match Hitchcock at his best, it does stand up well against a lot of lesser Hitchcock efforts. For example, it is better than "the Wrong Man," "the Paradine Case," or "Under Capricorn." and as good as "Secret Agent." The actors were all fine, with Christine Norden giving her usual better than they deserve performance. She was perhaps the sexiest women working in British cinema at the time.
Here's the basic plot setup. A married man and married woman leave their spouses and run away on a train together. Racked with quilt, the man decides to return to his home. It appears that the emergency chord on the train gets pulled and a train wreck ensues killing 10 people. The man soon becomes the chief suspect as the person who caused the accident, yet there is much more going on here than first appears.
Some people might find the style of the ending a bit of a disappointment, but I think it was actually pretty fresh in 1949, and not at all the cliché it later became when overused in later movies and television shows. It is more logical and more satisfying than most endings of this style.