Why did Elie compare the world to a cattle wagon?

In context, this is an example of irony. Elie says, "The world had become a hermetically sealed cattle car," but as we know, a cattle car is not sealed.... it is purposely created to allow as much air as possible to move through.... to keep its freight alive. In the case of the Jews, Elie is creating the paradox of the way that freight cars intended to keep their cargo alive were actually being sealed to allow the Jews to die.