In the end of Snowpiercer Wilford reveals that they use small children of the back for manual labour on the train. But if that is the only purpose of the people in the end why didn't he transform the entire back of the train into a breeding camp?
Well, that's pretty much what it was, wasn't it? The people in the back of the train had a miserable existence, they reproduced and some of the children were taken to perform the manual labor. You can't take all the children, because some need to grow to adulthood to reproduce. You can't kill everyone after a certain age, because eventually they'll catch on to what's happening and stop cooperating. Possibly even mass suicide. Then you have no workers from the lower class to run the train. So you keep it as a breeding camp, without letting them know it's a breeding camp.