the BEST ending would have been
the ending we see, and then this extra scene:
Jake and girlfriend are in bedroom, Jake says, 'this is just so amazing I can't believe it.' Gf says, 'I know hunny.' Jake then reaches to turn out light to sleep.
Scene cuts to doctor standing in front of Source Code. 'Well, until the next time we need you' he says, and presses button to switch it off. Last shot is of Source Code machine powering down and becoming dormant.