To me the big issue in the series was how children get abandoned, looks at what happens to them, the consequences of their fate on others and finally what it takes to change “fate”.
Fate – The train metaphors show how people tend to be stuck on a specific path in life. And it actually takes a jarring event to shift them on a different track; a new path in life. The use of burning and apples represents the sacrifice often needed to jar someone out of the direction, the fate, they were headed.
As an adopted child I can relate to the notion of sacrifice expressed through Momoka the most, and others like Shouma. That most of the core cast needed to be saved at one point. That in order to actually be saved, people from the outside have to make a sacrifice. Not spontaneous combustion which could be an allusion to a financial or some other sacrifice. Like my parents who never had a biological child of their own and had to pay a lot of money to get me here. If they didn’t save me I would’ve been sent to the Child Grinder, which from my perspective just represents all the awful things that happens to societies abandoned children.
The characters in the story represent a shit tons of different sorts of kids abandoned by society.
Ringo – The child of an over worked single mother and a “franchise” father. While the dad technically never abandoned her, the new woman and child he was with alludes to how many dad’s will abandon one family that “doesn’t work out” and start an entirely new one, leaving the mother and child to suffer.
Yuri – Represents a victim of child abuse.
Teacher guy w/e his name is – Represents a kid who failed to live up to his parent’s lofty standards.
Shouma – His parent’s abandoned him for the sake of some ideal.
Himari – A straight up unwanted and abandoned child.
Kanba represented the resentment of doing anything to protect those close to him at whatever the cost to those on the outside. He also symbollized the resentment and bitterness that led to the insider vs outsider, us vs them, mentality through what happened to him and his family.
The terrorist allusions represent how people disgusted with this world often lash out at innocents because the perpetrators are much harder to single out and punish. The pink haired dude represents this as a curse or whatever. That nebulous anger and rage that has no outlet and eventually causes only more grief in the end.
Finally Ringo and Himari represent the woman, girl, female issue. That in this world, girls often get it the worst, since most people are more tolerable of keeping male children. The fact that Ringo and Himari were the ones that survived, strikes at this issue.