"The Great Train Robbery" was the most successful film to date, and its distribution preceded and eventually coincided with the spread of Nickelodeons across America. It didn't instantly create the Western genre; instead, it was part of and led to a spew of crime pictures--a genre begun in England. (G.M. "Billy Bronco" Anderson, who was somewhat of an assistant director on the film, would largely invent the movie Western a few years later after leaving the Edison Company, however.) Although the plot isn't very exciting today, the film remains a landmark in film history--mostly for its narrative structure. It's also notable how matter-of-fact, "realistic" and violent the film is for its time--being detailed and rather objective in its following of the details of the crime (what Neil Harris calls "an operational aesthetic"). The story film was already established by 1903 but was still in its infancy. Filmmakers were still experimenting with how to tell a narrative, and Porter was one of early film's greatest innovators, as well as an astute student of film.
Throughout his film-making career, Porter was strongly influenced by contemporary British and French films, which he would have ready access to since the Edison Company regularly duped them. His "Uncle Josh at the Moving Picture Show" is a rip-off of Robert Paul's "The Countryman and the Cinematograph." "The Gay Shoe Clerk" is a revision of George Smith's "As Seen Through a Telescope." Porter also introduced intertitles to American cinema in "Uncle Tom's Cabin," having seen them in some of Smith's films. "Life of an American Fireman" reflects James Williamson's "Fire!" The temporal replay in that film was influenced by Georges Méliès's temporal replay of the Moon landing in "Le Voyage dans la lune." "Dream of a Rarebit Fiend" also owes plenty to Méliès.
"The Great Train Robbery" followed a recently created genre of crime and chase films begun in England. "The Daring Daylight Robbery" was especially of influence, as was, it is said by historians, a 1903 film called "The Robbery of the Mail Coach." "The Great Train Robbery" is also based on a play of the same title by Scott Marble. News of real train robberies also served as inspiration according to the Edison catalogue. Anyhow, with the exception of wholesale rip-offs, it's not discourageable that Porter learned from other films and adopted techniques and style from them for his own. He is worthy of history's praise for being such an avid student of film and one of the more active filmmakers of his time to develop film grammar.
Some film historians and critics say that Porter's work was uneven, that "The Grain Train Robbery" and perhaps one or two other films were a happenstance success, or fluke. Someone was bound to figure out the techniques of narrative as story films became more complex--confronting such problems as spatially separate actions and the continuity of action. I've seen a good share of Porter's work, however, and it's apparent that he was usually experimenting. He wasn't consistent like Méliès, which is good because his work becomes stale. Porter's previous experiments in editing resulted in this, his most accomplished story film and greatest success.
There are a few special effects in "The Great Train Robbery," as others have mentioned. It's nothing new: double-exposure matte work for the shot of the train outside the window and for the outside of the moving train's door, hand coloring, in addition to stopping the camera and splicing to replace an actor with what is obviously a mannequin. Most amazing about this picture (for its time) are its structure and the editing and camera techniques employed for its continuity. Panning and tilting wasn't new, but this movement of the camera in one scene to follow the action is exceptional for 1903. Likewise, Porter and others had already used the close-up. Porter employed an insert of a fire alarm in "Life of an American Fireman" and a privileged camera position for "The Gay Shoe Clerk." The shock value of the close-up in this film even serves form as its entirety is supposed to thrill.
Furthermore, the view from on top of the train is quite good. The transitioning between interior and exterior shots is fluent, and generally so is the continuation of action from scene to scene, with action exiting and entering scenes from appropriate directions. This is elementary film-making now, but in 1903, they were inventing it.
What I think is the most interesting part of "The Great Train Robbery," though, is its editing between the plot of the telegrapher and that of the robbers after their initial confrontation. After following the robbers for a while, the film cuts back to the "meanwhile" plot of the telegrapher. Initially, the barn dance scene doesn't appear to serve any narrative function--until the telegrapher enters to gather a posse. It's an interesting ordering of and transitioning between parallel actions. The plot isn't in temporal order, and it's a nice testament to Porter's innovation that a few modern viewers have been perplexed by how the posse catches up with the robbers so quickly.
It would take D.W. Griffith and others to build upon past work and their own in moving towards more entertaining and cinematic films, but the developments in narrative experimented with by pioneers like Porter paved the way.
(Note: This is one of four films that I've commented on because they're landmarks of early narrative development in film history. The others are "As Seen Through a Telescope," "Le Voyage dans la lune" and "Rescued by Rover.")
