So I did some more looking and it seems like this image is from a… basically a clip show of a “story” that’s going through the history of Superman.

So in that context we’re likely looking at a significantly less powerful Superman earlier in his history. Which also means that the situation was likely that the train itself was going to crash and he’d been pushing on it for much longer with the kid having wandered into the tracks while he was in the middle of stopping the train.