Edwin S. Porter made one of the most important early films "The Great Train Robbery". Some have referred to it as the first full-length film, though Georges Méliès made his "Voyage Dans Le Lune" the same year and a couple other films also claim to be the first. Regardless, it was a huge milestone in film history and was the first great American western. So, it's not at all surprising that Porter would seek to capitalize on this success--though I am surprised it took him two full years to get to "The Little Train Robbery".
"The Little Train Robbery" is a parody of his previous film--and it IS unusual for a person to parody their own work. In this case, you have a similar plot but it's acted by kids. And, as they are kids, they ride ponies and the train is an amusement park type they'd have for the young kids. It's a clever idea BUT it's also an idea that runs thin--even when full-length meant 10-20 minutes as it did in 1905. In many ways, it's a lot like the much later (and dreadfully awful) "The Terror of Tiny Town"--the first all-midget western! So is it worth your time? Not especially. But, if you love early films and have already seen "The Great Train Robbery", it's worth a look.