It may not share too many things with "Inception" outside of a slight similarity in means of quantum teleportation, but "Source Code" does have an ending that's worthy of interpretation, much like Christopher Nolan's (much weaker) hit from last year. In fact, some of the same theories applied to "Inception" could work for Duncan Jones' new "director-for-hire" follow-up to "Moon." I'll wait until after the jump to go into details, since obviously this discussion will include spoilers. But while I do believe Jones (and maybe to a lesser degree the original screenwriter, Ben Ripley) sets up a precise idea of what happens at the end, I've already encountered people with different thoughts on it.
So if you haven't seen the movie yet, do so, and then check if your interpretation matches mine below.
Here is the literal ending: Captain Colter Stevens (Jake Gyllenhaal), who has repeatedly been leaping into the body of a man riding a commuter train 8 minutes away from total destruction, completes his mission and saves Chicago from a dirty bomb. He then asks Goodwin (Vera Farmiga) to send him back to the 8 minute "loop" one last time so that he can also save the people on the train -- a feat deemed impossible by "source code" inventor Dr. Rutledge (Jeffrey Wright) -- and then at the end of the 8 minutes Goodwin is to pull the plug on the physical, comatose body of Stevens laying in the lab, thereby killing him. She fulfills both requests.
At the end of the 8 minutes, Stevens (in the body of Sean) kisses the girl (Michelle Monaghan) and time seems to freeze. Then it picks back up again and the couple goes out for their coffee and a visit to Cloud Gate in Chicago's AT&T Plaza and live happily ever after. Then, a kind of epilogue shows Goodwin arriving to work earlier that morning and receiving a text message from Stevens explaining that he has changed the course of the future by saving the passengers and that the comatose soldier in the lab will eventually go on to a different "first" "source code" mission one day. Hopefully that's as clear as the events can be laid out, though you also hopefully have seen the film so know what I'm talking about.
Firstly, here's the incorrect explanations that I've heard: Stevens is now split into two separate places at the same time,* because his consciousness is inside the body of Sean, like a "Quantum Leap" or "Being John Malkovich" kind of way, forever, whether he has control over the body or not (I guess because we see Gyllenhaal and only view Sean in mirrors, more like "Quantum Leap," Stevens is in control). And it is also inside the comatose, half-soldier laying in the lab awaiting his first mission. And none of this is a parodox in any way, because as the film acknowledges, the "source code" science may just be creating multiple alternate timelines/dimensions.
Or, all is the same but the Stevens spending time with the girl (her name is Christina) and the Stevens in the lab are in two separate timelines, because the text message created its own alternate reality. Or, all is the same but it is a paradox, because if the disaster was averted then no "source code" mission took place meaning nobody averted the disaster, etc.
But here's my explanation, which isn't too much different in terms of their possibly being multiple timelines, but I think it works even within a single reality, albeit a somewhat paradoxical one: Stevens returns to the train for its final 8 minutes and he succeeds in averting the disaster by handcuffing the terrorist and reporting his van full of explosives. However, when the 8 minutes is up, and the time appears to freeze, this is when Stevens would leap out of Sean's body and return to his own body.
Yet at that very same moment, Goodwin has terminated Stevens' body so instead of returning to the present he enters the afterlife. His afterlife looks exactly like the last moment of his consciousness, because this is what he wanted it to be. He held onto his perception of that last image and world and brought it with him to use it as a projection in his post-death consciousness, which is kind of like a dream. Actually, it's not much different from the projected surroundings he experienced in the present, the hatch and pilot garb he felt safe in while suspended in the limbo of the coma (or whatever state his mind was living in), and the explanation of that is where he got the idea that he could do the same in death, provided consciousness still exists after our body dies.
So the entire sequence of events from the moment that time on the train unfreezes up through to the Cloud Gate (and beyond, after the film ends) is like Stevens' heaven, as weird as it may be for him to still see someone else's face in the mirror, and all the other people are a part of his imagination. As I said, this sort of afterlife is like a dream, and so the other people, including Christina, are just subconscious projections and not the real deal. I'll admit, this is how I expect the afterlife to be, like an eternal sleep/dream.
Meanwhile, somewhere, the real Sean is alive, and maybe is even having coffee with the real Christina, but there is no Stevens inside his head. And the epilogue is in the same world as that real Sean and Christina wandering around either together or apart. Whether it is a paradox or an alternate timeline from the original one in which Goodwin sent Stevens back and then ultimately terminated him is not that important. Unless you want to assume that Stevens isn't dead and is currently dreaming his date with Christina while laying in the lab, and that moment of time freezing merely rebooted him back into the lab anew. Regardless, he's not in two places at once.*
And hopefully all that makes sense, whether you agree with it or not. As always, these time travel movies and their quantum-based explanations are tricky and will likely require some further thought and repeat reading and viewing. For now, though, let's try to work out other thoughts down below.
* Because of the confusion, I must note that by "two places at once" I mean within the same dimension/reality/timeline and didn't intend to dismiss the parallel worlds idea. But even with that said, I might have been confusing myself. He could technically be in two places at the same time the way there are simultaneously two Martys in 1955 in "Back to the Future," due to time travel. I just mean his consciousness itself could not be split in half. Anyway, I'm giving myself another headache.