Add to this that the child is also made entirely of rubber and could easily withstand the train's impact and experience no measurable hardship. However, the impact of Superman halting the train caused wreckage to fly all over the place and damage the surrounding infrastructure.... which in this case is a metaphor for literal fucking infrastructure.
And killed the driver (who I think is called and engineer but I'm not sure and I'm not going to look it up because I don't feel like but even though it would take me less time to actually check than to write this explanation but to hell with it) who represents the middle class.
It's the trolley problem. Picking up a kid in time to beat a speeding train would just snap his little neck. Stopping a speeding train to save the little boy would most likely kill anyone on board.
This scenario indicates superman would rather kill a train full of people, rather than having witnesses see him sit idle as a child removes himself from the gene pool.
My hypothesis is that this is the child of Lex Luther, or possibly a clone. He's literally playing on train train tracks, baiting super man into a win-less scenario via the trolley problem. It's the perfect scheme to slowly make him question the consequences of all his actions and eventually his entire purpose.