

Him starting off at the velocity of the train is the problem. It’s not as simple as “Superman just goes a little faster,” momentum has to be conserved. To launch himself forward he has to launch the train backward and the train likely doesn’t survive that. Continuing to push the train was still likely the best option Superman had in this situation.
You’re ignoring the part where I said this seems to be a memory of an earlier point in comic history for Superman we’re seeing. This version of Superman is almost certainly nowhere near that fast.
He also doesn’t need to weigh more than the train for the train itself to not be able to withstand the force of him pushing off.