There are all kinds of ways of being trapped; it's not always physical. Characters can be restrained by physical bonds, laws, or words. In Water for Elephants, we get a little bit of everything in the trapped department: Marlena is legally married to August; Jacob is physically held back by people like Blackie and Earl; Al forces Jacob's hand by threatening Walter and Camel; the list goes on.
And then of course there's the confinement of the animals. This is a huge controversy when talking about circuses, and although Water for Elephants doesn't address it correctly, there is a subtle discussion. Shmoop thinks the takeaway is this: confinement (of animals and otherwise) is probably going to backfire. Think about it: Rosie actually uses her boundary as a weapon, pulling the stake that's supposed to tie her down out of the ground and using it to kill August. Yikes. Are there other times in the book when confinement turns out to be dangerous?