There's not much to this part except to escape from the infected! You'll have to help Riley out by opening the shutter in the mannequin room and then put a cart underneath it. Quickly continue to follow Riley through the halls; the rest is rather self-explanatory. Enjoy the ending scene of the game!
Note: You'll also get the 'Don't Go' Trophy for all difficulties you've completed the game on (so you'll also get Easy and Normal, for example, if you've finished on Hard).