When the train comes out of the tunnel, Britney and Nick are sitting as if nothing had happened, and Justin is holding his slapped face.
Justin is thinking, " Nick must have kissed Britney and she swung at him and missed, slapping me instead."
Britney is thinking, "Justin must have tried to kiss me accidently kissed Nick, and then got slapped for it."
Nick is thinking, "This is great. The next time the train goes through a tunnel, I'll make another kissing noise and hit that asshole Justin Timberlake again."