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.