According to the Hollywood Reporter, Michelle Monaghan will appear with Jake Gyllenhaal in Source Code, and Vera Farmiga is also in negotiations to join the cast:
Farmiga will play a hands-on communications officer controlling Gyllenhaal as he travels through time and space. Monaghan is a woman on the train with whom the man is involved romantically.
(Photo strip shamelessly stolen from The Film Stage. It's too late for me to try to make one of my own!)
Not sure how I feel about this. I wasn't crazy about Monaghan in Gone Baby Gone. And she doesn't match what I've heard about the character. OTOH, I loved Vera Farmiga in Up in the Air and The Departed, so I'm happy about her involvement.
A blog also posted some cool photos of the train station that will be used for filming: