The Great Train Robbery is a film that has a rather significant place in film history. Contrary to popular belief, it is not the first film to tell a story (it was preceded by The Life of an American Fireman and A Trip to the Moon), but it did build on its predecessors and establish more sophisticated editing techniques than had been used previously. Ultimately it would help to make a path for D.W. Griffith's Birth of a Nation which in turn made possible every film that has been made since. That last shot alone became notorious back in 1903 when its first audiences actually expected to be shot.
Of course, that is likely what you're initially thinking of when you hear that title. What you may not realize is that this is not the only film about... well... a great train robbery. Several decades later, another film by that same title was released, directed by none other than Michael "Jurassic Park" Crichton and based on his own novel. Like the 1903 film, it was loosely based on real events, and centered on a plan to rob a moving train. That's pretty much where the similarities end.
During the Crimean War (1853-1856), the British government decides to pay its soldiers in gold. That's all well and good but they also realize very well that doing so might attract unwanted attention. They devise a sophisticated plan to ensure that the gold cannot be stolen: first they transport it by train (the film notes that nobody had ever successfully robbed a moving train at the time), with a safe built with four locks and four seperate keys that are kept by different people, all of which are required for it to open. Everything seems to be perfect.
Enter a criminal mastermind only referred to by the aliases "Edward Pierce" and "John Simms" (Sean Connery), who pretends to be a successful businessman all while collecting information in order to devise a plan to steal the gold. To open the safe he enlists Agar (played by Donald Sutherland with an awesome mustache), a forger who agrees to create replicas of the keys if they can be obtained. His other partner-in-crime is his beautiful girlfriend Miriam (Lesley-Anne Down), a former actress and master of disguise who knows how to use her feminine charms.
This film is inevitably a lot of fun. The period detail is extraordinary and the pacing is very good. There is lots of intrigue surrounding the planning and execution of the heist and how it's all going to go down. Along the way various challenges present themselves, but it just becomes even more interesting how our heroes figure out a solution. When it seems impossible they have to think outside the box. When they don't have the proper means to get through they have to figure out how to get them.
That's basically the structure of the film. In short it's a step-by-step process, where our daring heroes have to take care of each obstacle, with individual segments dedicated to obtaining copies the first two keys plus a third for the last two (which are both locked up in the same place), plus any further information and obstacles that come up. Eventually it all culminates in an exciting climax with lots of daring stunts and an ending that leaves you to question just how much was planned before and how much was improvised along the way.