The idea is he’s straining so hard because he’s trying to use the exact amount of force he needs to stop the train without harming anyone inside. Too much force on the train and everyone in the train gets injured or killed, too little and it doesn’t stop in time to save the kid, and I believe in this one he didn’t have the option of just grabbing the kid because he would have been hurt too badly from the sudden acceleration.

If you’ve ever tried to assemble something where you’ve gotta snap together two pretty fragile pieces it’s a similar idea. You absolutely can generate way more than the force needed to get the job done, the difficulty is in having the pieces survive the attempt.

I can tell you I have experienced it with models and computer components and you’d absolutely think I was arm wrestling a God with how much I was straining trying to push those parts together without breaking them.

Edit: I’m not sure which comic this is actually from but when trying to find what issue this was from I saw people saying something about explosives on the train too? Not sure how accurate that is but the idea goes to support the usual thing with writing Superman that he often doesn’t have an easy resolution to whatever crisis he’s facing.

If the explosives thing is accurate then taking the time to move the kid means the train likely gets closer to more people an explosion would hurt. Would it be enough to matter? Maybe not, but Clark would pretty reasonably believe it would.