!SPOILERS! Connected parallel universes!

The main point of the ending is for the viewers to "realize" that, when someone gets sent into the "Source Code", it creates an alternate reality.

This "new" reality is exactly like the "original" one up until the point where the protagonist starts making new decisions.

At the end of the movie, when the protagonist sends the SMS, it's a new reality / timeline. He thinks he's going to die after 8 minutes so he just tries it anyway! He then disables the bomb, arrests the crimimal and kisses the girl AND THEN Goodwin receives a text message, in this alternate reality, where no explosion happened.

I rather would see it this way: each time the source code is activated Colter creates another alternate reality, which coexist next to each other. So there is the reality: no explosion, Goodwin gets the SMS from Sean and the real Colter is lying in his box waiting that the source code will be tested for the first time - and next to this reality there is the reality: train-explosion, Goodwin gets no SMS and finally shuts down Colter.

The other possibility - the source code enables real time traveling and each activation overrides the last one does not make sense in the movie - otherwise there would be no scene where they celebrate that the source code worked, as in fact - and next to this one and only reality - no train exploded and the source code was not tested.

So there is only the 8min where the parallel universes are connected - the source code project is activated and by pressing the button & another parallel universe is created.

This is heavy.

I rather take it as it is myself, so to speak. Yes, it can be convoluted but I personally like the direction it took.

Source Code came out just after Star Trek 2009 made alternate realities cool so yeh it was all to do with alt realities even though Felix Leiter said it wasnt