I reallly love that song but I dont know what its called. its the part where blair and chuck are in the train station and she has that red dress on.

B.o.B “Ghost In the Machine”

Chuck heads off to work & runs into Blair; repeats as Serena answers Blair’s text; Juliet tells Nate he’s not ready to date; Rufus figures out that Milo is not Dan’s son; Juliet takes the photos off her bulletin board; Chuck asks Eva to go to New York with him.

http://www.youtube.com/watch?v=oQ17YV5obwI

Other songs featured in this episode -

“I’m In Here (Piano/Vocal Version)” by Sia.
Eva waits for ‘Henry’ as Blair & Serena arrive at the train station; Blair tells Chuck he owes it to everyone he’s leaving behind not to run away

“Gone” by Tokyo Police Club.
Blair pesters Serena about not choosing between Nate & Dan, then she heads off for a date with the prince.

“This Loft Is My Loft (to the tune of ‘This Land Is My Land’)”.
Nate & Juliet find Dan singing to baby Milo.

“Last Flight Of The Bat” by Goodbye Motel.
Vanessa & Juliet return from shopping.

“Je Ne Te Connais” by Prototypes.
Serena tells Blair about her visit to the morgue, then Blair tells her about seeing Chuck.

“Je T’amie… Moi Non Plus” by Serge Gainsbourg.
Blair tries to impress the prince with her simple tastes.

“Beat The Devil’s Tattoo” by Black Rebel Motorcycle Club.
Blair is invited to Harry Winston on the prince’s behalf; Serena tries talk to Blair about Chuck.

“9669″ by The Joy Formidable.
Serena tells Blair about Chuck’s intentions and that she’ll regret it if she doesn’t stop him.

I hope this helps!