If you're incapable of understanding the basic premise of a film without tediously referring to it as "some other film with a vaguely similar plot element meets some other film with another vaguely similar plot element", despite the fact that the film you're talking about is clearly nothing like either of those other films, then you're in luck! The internets are here to tell you what to expect from Source Code.
"Groundhog Day meets Déjà Vu"
"A mix of Inception and Groundhog Day"
"An intriguing Groundhog Day-style premise"
I don't know about you but I feel like I'm experiencing déjà vu on Groundhog Day.
Here's a promise. If Source Code turns out to be even remotely like Groundhog Day I will literally go out, buy the biggest hat I can find, roast it in beef stock and red wine for six hours at gas mark 5, sprinkle with herbs, add a little salt and pepper and ram it up my arse.