Did Source Code actually change reality, or did it create an alternate reality, or...?
When Goodwin took Colter off of life support, my initial thought was that, by letting him die at that moment, his brain could continue in the alternate reality for another 8 minutes. In retrospect, that doesn't really make sense, considering the information we were given by the creator and Goodwin. I definitely need to watch it again, though!

Also, alternate realities and time-travel aside, I think Source Code would be an excellent TV show. It would probably only be good for a single season, but they could use the basic premise (reviewing the last 8 minutes of someone's life) to do some interesting things spread across 13 or 22 episodes.

I think an even better ending would be if it had ended at the freeze frame. Why didn't Jake Gyllenhaal try and track down the real Sean? Does the ending mean there are TWO Seans and he is living his life (inside the teacher's body?) with Christina. The film could have been a bit longer IMO, as he found the bomber too quickly. Why didnt he try something like talk him out of bombing the train?

I'm so increidbly glad they didn't go the "talk him out of it" route, that is so damn unrealistic and contrived. You don't to the trouble of rigging a train, blowing it up next to a tanker, and building a dirty bomb only to be talked out of it by a partially dead soldier.

And no, there are not two Seans. There is only one, it means he basically went to sleep as Sean and woke up as Colter, so Sean is either sharing the same consciousness as Colter from that point on or Sean was wiped out completely when Colter took over

Scott Bakula providing the voice of Colter's dad and uttering "Oh Boy" at the beginning of the call.

Fair do's that they didnt "talk him out of it", I thought he found the bomber too easily though. I thought there'd be a twist that Michelle Monaghan was the bomber or something.
Also if we go with the "other universes" theory how come Sean/Colulter remembers things each time?

I think Christina being the bomber would have been way too obvious. Right from the first time Sean/Coulter wakes up, she had a little bit of a creepy smile on her face.

I don't think he necessarily found the bomber too easily, though. Sure, in terms of what we saw on screen, it seemed to happen fast, but we don't know exactly how many times he went back into Source Code.

I can't answer your question about how he was able to remember things each time he re-entered, but I'm sure there's some sort of reasoning.

there was going to be a twist where it ended up that the body he was using was the bomber.

didn't Goodwin say something like "You yourself is the key"?, I thought that meant Coulter would try and track down "another" Coulter.

That and also the part where she said he came out of the bathroom.

considers himself as a suspect

I thought it was sorta unfair that he took over the guy's body at the end. But I suppose that guy was dead anyway in the real reality. The guy that ended up being the bomber was my very first guess while watching it. However, they threw up many red herrings that made it interesting.