The departure of people doesn't seem to be as sporadic as everyone else is determined to make it seem. Too much drama. Actually, people depart at certain periods, usually in groups, more often than not. And whilst I don't pretend to know the personal lives of everyone on board, it seems as though people leave after a certain period and, as we all know, no one here has a dull life outside of the train. They're going back to their reality to do whatever it is they need to do, with the bonus skills that they learnt on the train. Of course people come back and say they don't remember, but we don't know that they don't remember when they get home, we just know that they don't remember when they arrive on the train. It could be that the train removes memories from the train as part of it's make up because then when you go on holiday it's new all over again, so arriving on board you forget what you did the last time you were here. But when you leave of course you remember, you can't enjoy a holiday if you can't remember it. So it seems that when people go back it is to do something in their own reality, or because, perhaps, the train thinks that they have gotten all they can from their time on board. Or perhaps they're too distressed and there's nothing it can do. Your memory loss was distressing, but it taught us that we would have been together anyway. Thor disappeared but he had a kingdom to rule. James disappeared but he had either made peace with dying or he had decided to get some more out of life. Peeta left, but the train was too distressing for him and nothing good was coming of it. But we are from the same time, we still have things we need to do here, clearly, and London isn't missing us. There's no benefit to sending us back and we're okay where we are. People don't look outside of the immediate box to the reality they left.