Enter the waiting room and use the bell to call the usher (notice an infusion located behind him). The salesman won't be particularly interested in your person and this is a moment when you have a chance to make a new decision: you can draw your gun, ask for the tickets or restrain yourself from doing anything. Depending on your choice you can expect Elizabeth to act differently at the end of this chapter. If you decide to ask for the ticket, then Booker's hand will receive a wound resembling a stigma.
It doesn't really matter how you've reacted towards the usher, because you'll soon find out that you've been ambushed and that people gathered in this hall want to capture the girl. Elizabeth will run to the gondola and you'll have to fight your way out of this area. Try to deal with the thugs as soon as possible, because reinforcements will soon use a previously unaccessible door to get here. Once the battle is over search the area and then proceed to the opened gates. Elizabeth will escape towards the gondola.