I had the good fortune to see Source Code while knowing nothing at all about it in advance, other than it having "something to do with sci-fi."
And it began in a train with a shot of a giant-headed white guy turning from the window and looking strange, as though he'd just woken up or just appeared there.
He stood up, and I said to myself, go to the bathroom! And look at yourself in the mirror! He didn't. He stepped outside the train.
But then he returned to his seat. And he got up again! And went to the bathroom! And looked in the mirror and had a revelation!
I can say all that without spoiling any of the plot. But could I go further and explain lots more that links with Lost? Yes.