"The Source Code technology in the film is inspired by real-life quantum mechanics theories, which posit the existence of multiple parallel universes and alternate timelines." https://repeatreplay.com/source-code-movie-ending-explained
This is such a fantastic movie. Source Code isn't so much about time-travel (TT), as its more about linking one's consciousness to a parallel world. Thus, at the end of the movie it is revealed that the protagonist is in another person's body, which is fully intact on an alternate world.
"Stevens presses him for answers, he's told that Source Code isn't a simulation, as he suspected, but an extremely limited type of time travel. Rutledge clarifies that it's more like time re-assignment, and it works via quantum mechanics and parabolic calculus." https://www.looper.com/1284014/source-code-ending-explained
https://thecinemaholic.com/source-code-ending
The Nightmarish Delirium of SOURCE CODE Explained https://www.youtube.com/watch?v=IktseUs3TZg
Source Code Movie Ending Explained + Multiverse Explained https://www.youtube.com/watch?v=-zLX0SmD1bM
There is a touching scene between the protagonist and his father. Its an unofficial connection to the protagonist in the original Quantum Leap TV series.
