Colter Stevens: What would you do if you knew you only had one minute to live?
Christina Warren: I'd make those seconds count.
Colter Stevens: I'd kiss you again.
Christina Warren: Again?
Colter Stevens: Tell me everything gonna be okay.
Christina Warren: Everything's gonna be okay.
Christina Warren: I took your advice. It was a very, good advice.
Dr. Rutledge: You cannot alter this reality while inside the source code.
Dr. Rutledge: This is not time travel. This is time re-assignment.
Colter Stevens: It's the new me.
Christina Warren: I like it.
Colter Stevens: It's the same train, but it's different.
Answer: He died in the train bombing along with all the other passengers. If you're asking what happened to the real Sean Fentress in the alternate time line created at the end of the movie, Colter took over his body and continued to live the life that Fentress had been living.
Casual Person