The reason Source Code works for me while a movie like Deja Vu doesn’t is because it’s drawn using clean straight lines with the kind of geometric precision and simplicity that Borges’s stories are known for. Unlike Deja Vu, it doesn’t weigh itself down with cluttered dialogue and clunky science in validating and explicating its assumptions. It isn’t science fiction. It isn’t interested in the science (The only line of scientific gobbledygook in the movie, “Quantum Physics and Parabolic Calculus!”, is delivered by the ‘scientist’ in a tone that is more smirk-y than matter-of-fact). It’s a fantasy spun around an idea. Source Code poses a simple question, and without much ado goes ahead with answering it with a gripping sense of urgency. The complexity arises in contemplating the answer, and like all great works of fiction this is (implicitly) left as a task to the interested reader (read viewer). The film it most resembles in plot is, of course, Deja Vu; in construction and existential ramifications, Groundhog Day; but in metaphysical spirit, it is the digital motion-picture equivalent of Borges’s The Garden of Forking Paths.
PS: Why couldn’t they come up with a more imaginative title?