What does the final ending of the movie source code mean?

I feel like I don't understand upstairs. This is what I understand. Please don't spit it out.

Doctors and female captains think that the source code they created can only last for 8 minutes and cannot change the past. But this is not the case. Not the so-called worlds A and B. Look carefully, there is a detail. In the 66th minute of the film, the hero asks the female captain if there is another version of her if she is not married or divorced. The female captain interrupted him and said that what the man saw was only an illusion. The technology of source code was not used to change the past, but to influence the future. But this man firmly believes that this can be changed. He repeatedly said that he would go back to the train to save the passengers. They don't have to die, and they say that the female captain is wrong. In fact, by the end of the story, I found that the man was really right. Because the female conductor sent him back to the train, she saved the passengers and arrested the criminals. When he kissed Chris Ding Na, he thought that the beautiful environment he created would be over in eight minutes, but no, time went on. Everyone was saved, and it was not in the source code world (because the source code world can only last for 8 minutes) that he discovered that this technology can actually change the past. He really saved those people and sent a text message to the female conductor saying, "You and I will stop the explosion together." That's the point, because he persuaded the female conductor to let him go back to the train to save people. Also, if they are in two different parallel worlds, the female captain can't receive the message from the male host. So it can be proved that they are in the same space. The explosion did not happen. At the end of the message, it said, "Goodwin, if I am not mistaken, somewhere in the source code equipment, there is a captain named Colt Stevens who is going to stand by and promise me that you will help him and tell him that everything will be all right." It means, let the female captain help him go back to the past to save himself. In order not to let him die in Afghanistan. Some people may say that since he can send a text message to the female captain and he is not dead, why save himself? Because it was Sean who texted, not Captain Colt Stevens. At the end of the movie, when the man and Chris Ding are in front of the Chicago mirror (= = I don't know what that is), Byakki Smoker is Sean and his body is Sean. So when he discovered the real secret of the source code, he wanted Goodwin to help him save himself and then go back to his father.