I would argue that one of the most significant happenings that occurs on the crowded train that has direct impact on Jacob, and indeed the whole plot of this great story, is the sickness of Camel after consuming poisoned alcohol and then the way that both he and Walter are redlighted, or thrown out of the moving train and killed. Let us remember that "redlighting" was a way for circus managers to get rid of unwanted bodies and probably killing them as they were thrown at high speed from moving trains. Camel, because of his incapacitating illness, obviously was just another body to feed, and therefore, in Big Al's eyes, was not economically productive. In such a harsh world as the novel presents us with, Big Al had to be very strict to make a profit with his circus, and we can see Camel and Walter become tragedies in Big Al's relentless determination to maintain his power and success.
This is the central event that forms a subplot to the romance between Jacob and Marlena, and it is this that prompts the chaos and retribution in the final scene as the animals are loosed from the menagerie.