If the parallel universes already exist then why were the exact same people on the train each time, one might have been sick or dead in one universe? Unless that is the exact point of change where they become different universes as he exists in one and not the other.
If they created a new Universe each time then they killed off millions in all but about 2 of them each time and how do the people just suddenly exist in those universes at that stage in their life?
I like the theory that the Universe ceased to exist each time after the 8 minutes until the final one where he joined an already existing alt universe.
Every explanation just leads to more questions.
Just to add: I didn't know the cast apart from Jake and thought Goodwin was played by his sister Maggie at first.