Source Code takes its inspiration from the best episodes of The Twilight Zone and The Outer Limits: ask a "what if" question about something just beyond the edge of our scientific understanding and explore the effect it has on the people involved. In this case, the "what if" sends an earnest and initially confused Army pilot on a new mission exploring the last memories of the victim of a terrorist bomb. Except he's not as convinced as the scientists that nothing he does will change the outcome of that explosion.
As he repeatedly experiences the victim's last eight minutes and repeatedly fails to either change the outcome or fulfill his mission to uncover the bomber and prevent another, more brutal, bombing, he becomes more and more convinced that he can make a difference for this train full of people. Even so, the audience wants to believe he is right, but becomes more and more convinced that the scientists know what they're talking about (even though the science is just as preposterous and under-explained as in any science fiction story).
Good acting. Good directing. Good suspense. The same big explosion seen from several angles. Decent special effects.
Source Code (IMDb)