I'm actually okay with it not making sense, a problem that many people identified with the film. It works pretty well even though the ending doesn't entirely compute. It's just the slapdash lameness of the final line, the philosophies that the film decides to take on for the lack of time to explore more interesting ones, and the way that director Duncan Jones and his writer Ben Ripley seem to be struggling to find somewhere to pull out that make any such rationalizations useless.
The same thing happened with Jones' first film, "Moon," an extremely astonishing achievement of atmosphere and ideas that lacked the execution to work those things into a movie just as great, largely because of the confusion of its climax and ending. "Source Code" is throughout a better carried out film, and it uses many of "Moon"'s strengths (making the audience claustrophobic and thus creating an intense interest in the mechanics of the plot) to its advantage. But it has plenty of its own missteps.
Jake Gyllenhaal does work well in the lead role, applying a mix of incredulity, manners, and sarcastic menace to his character. However, he is somewhat shaky at times, as he is not an exceptional actor by any means, and his limitations don't let the movie get to where it could have gotten. He plays Cpt. Colter Stevens, who has gone straight from being the military to waking up on a train. He stumbles around, wondering what the hell is going on, since he's (apparently) been placed in medius res into an operation where he's taken someone else's body to find out who put the bomb on the train he's on, a man who has more attacks ready to go. Since he has no idea what his objective is the first time through, he cannot complete it, and only goes to alienate the woman he's traveling with, Christina Warren (Michelle Monaghan). Monaghan, who plays surprised often in this film, gives the film's strongest performance, making the contact she shares with Colter (or, as she knows him, Sean) feel like something to behold. She's what makes the film move through its weaker portions.
Maybe her character is more interesting because she's given in small doses, but in any event Monaghan is only in half of the scenes, and in the other half of the film, we are on our own. That's because we have to deal with the interplay between Gyllenhaal and Vera Farmiga, who fails to realize a potentially compelling role due to one of the dullest acting jobs I've seen in a while. I guess it's like that by design, but that doesn't change the fact that I dreaded her scenes. She plays Goodwin, one of the people behind the mission that Colter is undertaking. She is affectless and talks and behaves as if she hasn't been outside in a long time. No wonder she says she got divorced. She's joined by Jeffrey Wright, who enunciates words strangely and who isn't successful in making his crutch look like anything more than a slapped on character trait. He's there to sound cool explaining big concepts, but he's barely less stodgy than Farmiga. It doesn't help that the interior decorator of the Source Code facility hasn't upgraded the place up to the current digital era. It doesn't seem like anyone has heard of an Apple computer or a wireless keyboard. Perhaps this is done to both add analogue flavor and to further concentrate the suffocating spaces on us, but it doesn't work too well, and it just goes to underline even more that this is much boring section of the film.
So perhaps Gyllenhaal should be credited for livening things up a little bit thereabouts, along with the help of some nice compositions by Don Burgess, strong editing by Paul Hirsch (especially in the train section), and a (sometimes more than) serviceable score by Chris Bacon. But "Source Code" is another near miss by Jones, who has now twice danced near making a masterpiece but bowed out both times because he hasn't tightened the bolts. B-