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.