In this interview with director Duncan Jones and writer Ben Ripley, Ripley explains that each time Colter goes back he enters a parallel reality (so he is not changing the past, he is accessing the last 8 minutes of a different reality experienced by Sean Fentress). Inhabiting Sean’s body/mind, he learns a bit more which he can take with him on the next “trip.” The conundrum is whether these parallel realities already exist or are they being created by the act of sending Colter back? In the end, he goes into a reality where he is the one that stops the bomber, which in a linear world, connects to a different future.
And I think that Source Code is different in that it says if
something happened in our past, you can’t change it. And that’s in
line with what theoretical physicists are saying these days. You
can’t change it, but what you can do via Source Code, or some other
technology, is access a kind of parallel track. Now, was this
parallel reality there to begin with always, and we just kind of
opened a portal? Or by virtue of sending him there did we actually
create it? I think that’s actually, even headier. And the end of the
movie, when she gets the text message, implies that Source Code‘s much
more powerful than you realize. Vera Farmiga, we actually created you
by sending me back on that train.
...we reset and we rejoin the story in a parallel reality. One where
a train has not blown up on it’s way to Chicago, but (one where) poor
Sean Fentress is now inhabited by Colter Stevens, and there is a body
waiting to be sent on a mission at Vera Farmiga’s facility.
Did Rutledge know what the Source Code would do? Ripley refers to Rutledge as “the bad guy who’s just obsessed with the technology he created.” I think that indicates that he did know.