Source Code (2011)

There is a concept that all media relies on, called “Internal Consistency.” 
It’s the premise that, if the story needs it, 2+2=5. It’s a building block that helps define how the world being built will play out; you’re already familiar with this idea: Jedi have Force powers, the Targaryens are crazy, and Adam Sandler is somehow someone that other people tolerate.
All stories have to build their own internal consistency, even if that includes ‘not’ building new rules by simply being a fictional story set in the ‘real’ world where 2+2=4 (ie: if your story is set in ‘real’ 2019 NYC, you wouldn’t spontaneously include Big Foot).
They don’t necessarily need to be explained, just developed and followed.
The best stories are the ones where those consistencies are built and you don’t realize it, then followed throughout.
The worst ones make it painfully obvious that they’re building their consistency, then decide that it’s not convenient half way through and absolutely shit all over their own premise to reach whatever conclusion they want.

“Source Code” (2011) is of the latter category.
The plot device that this movie is based on is that there’s a special research center at Nellis Air Force Base that utilizes a program called ‘Source Code,’ which is some vaguely defined Venn-Diagram overlap between “quantum entanglement” and “your short-term memory is always tracking the most recent eight minutes of your life.”
By using physics(?), scientists are able to send the consciousness of an Army captain to relive the last eight minutes of life of a man who died in a train bombing in Chicago that morning.
According to a monologue from the lead researcher, Dr. Rutledge (Jeffrey Wright), Captain Colter Stevens (Jake Gyllenhaal) was a perfect match for the man who’s mind he filled, because... they were both white males of average height and weight.
Super technical guys. Good job.

The ‘source code’ program somehow allowed Stevens to not only relive the last eight minutes on the train, but interact with the environment in new ways. He could talk to other people, get off the train, act like a crazy person, etc. Every time the eight minutes were up,  he’d be brought back to reality, debriefed by an Air Force captain, then return to the event to look for more hints about who set off the bomb and why. In this context, the eight minutes that Stevens was living through were the last eight minutes before the train exploded and the ‘host’ body was destroyed.
The movie takes painful measures to tell you that he only has some weird 4th-wall-breaking ability to “look around,” based on the “code” of the universe (all Matrix-like), but it’s pretty clear that he’s actually influencing what’s going on.
This was all enabled by alternate realities. Or a computer. Obviously. 
It’s unclear if the researchers at Nellis were aware that they’ve somehow managed to breach the membrane between realities and are actively influencing other ones, or if they think that it’s somehow just a super-powerful simulation program that relies on quantum physics.
And another catch: if just a simulation, there’s nothing that Stevens could have changed, so everything they’re doing is pointless. If we’re seeing a bunch of alternate realities, then we’ve just seen a half-dozen times when everyone on that train died and nothing is different in those universes, so… everything is pointless. 

Around the middle of the story, we find out that Captain Stevens is actually just a brain in a jar. He was a combat helicopter pilot in Afghanistan who died, sorta, and apparently Dr. Rutledge decided Stevens’ brain was the perfect one to plug into a system that had never been tested.

With Stevens apparently being the first Guinea Pig of this program, Dr. Rutledge made two things clear: 
1. That Stevens could not change the past, even if/when he solved the bomb problem.
2. That there were only eight minutes to live through.
He said both of those things multiple times.
He might as well have looked straight at the camera and said “These are the rules this movie is based on.”

However, during one of Stevens’ repeats, he gets out of the train at a stop, the train explodes, and Stevens keeps living. There’s a contrived death a few seconds later, but the ‘end’ of that eight minute cycle should have come when the train exploded, per what the audience is shown in every other repeat. So they broke rule #2 right there.
Stevens might as well have just walked away.

The end of the movie comes when Stevens finds the bomb, disarms it, and manages to subdue the bomber. There’s some mild celebratory stuff and Stevens kisses the fairly-irrelevant love interest, Christina (Michelle Monaghan). Then the scene stops in a freeze-frame/camera pan/zoom out as the USAF Captain turns off the machine running the code and keeping Stevens alive.

That was the perfect ending. 
Except they kept going. 
For 10 more minutes.

For some reason, the director decided to shoot for the “happy” ending, where no one dies and Stevens and Christina happily get off the train and start a budding romance. 

Reminder: Stevens is in someone else’s body, so whomever Christina knew is now effectively dead, and Stevens has to completely pick up the life of the man he replaced.
This is the issue with every body switch/snatcher movie ever: just because you look the same doesn’t mean your mannerisms will match, and whatever it is about “you” that your love interest loved, is now gone. Good luck filling those shoes.

The ending is further ruined when Stevens figures out the contact information for the USAF captain and sends her an email, somehow hours before the train bombing.
At no point in this movie was excessive time travel an option, as it was clearly limited to filling someone’s consciousness for those last eight minutes. This also has the added effect of completely mitigating the events of the movie, since apparently they didn’t happen, so now you have all the fun of a broken recursive time-travel loop.

This movie had a “Groundhog Day”/“Edge of Tomorrow” (1993 & 2014, respectively) feel to it, and used the exact same plot device from both yet executed it in the worst way possible.

There was also a lot of stupid “military” stuff included, making it pretty clear that no one on set bothered to reach out to the either the Air Force or Army Public Affairs teams for clarity or information.

Do not waste your time watching this.