The movie focuses on one mission: a Metra train heading into Chicago exploded, and Jake needs to find out why. The movie starts out very repetitive: he finds himself on a train, talking to a woman. After eight minutes, the train explodes, he dies and then it starts over. He tries multiple people and multiple strategies, and eventually figures out who bombs the train. The biggest problem is that it kind of feels like watching someone play a videogame. Watching other people play games is boring. If it were actually a videogame, then it might be kind of fun to talk to the people, starts fights with no consequences and solve the mystery. Also, there could have been more levels than just the train level.
What I didn't get, however, was how the source code machine actually worked. They said it put him into a simulation of people's memories, and that it wasn't a time machine. Assuming that is the case, then he would only be an observer. It's not a time machine, so he can't alter anything. He would have to observe the same things as the body he was projected into saw, and that's it. For example, at one point, he gets off the train, goes into the station and picks a fight with someone in a bathroom. The dead person he's in had no way of knowing that person was in that bathroom, because he stayed on the train. At that point, how do we know that the things he's seeing really happened and aren't just his brain making things up, kind of like a dream? It doesn't really make sense to me, so I went to IMDB to try and find out, and this is what I found:
It could be clarified that each particular *instance* of a consciousness can only exist in one reality at at time, but the Source Code device can project a particular instance of a consciousness from one reality to a parallel one--causing multiple instances of that consciousness in the destination parallel universe, as happens in the movie.
At the end of the movie, reality B contains *two* instances of Coulter's consciousness--a new instance of Coulter's consciousness, projected into Fentress' body from parallel universe "reality A" by the Source Code device, and reality B's original instance of Coulter's consciousness, in his ruined body in the Source Code chamber. In reality B, the train bomb never detonates and the dirty nuke is seized before it becomes a threat, so its original instance of Coulter's consciousness in his body the Source Code chamber is never called into action, and remains ready for a sequel.
So basically, he changes things in alternate universes, and saves all the people in the alternate universe. The author of that rather lengthy explanation of the movie doesn't cite where in the movie any of that is explained, so I didn't think it's explicitly stated in the movie.
Without that explanation, the movie doesn't really make sense. With it, it just seems like a live action video game that would have been much more fun to play. The ending doesn't really leave anything open to a sequel, which is good because this movie wasn't that great.