One idea would be a sort of controlled-access staging area before the train that would admit roughly the capacity of the next arriving train. The doors to the staging area close as the train is arriving or the staging area is full, and the doors to the train don't open until the doors to the staging area area confirmed closed. It doesn't stop people from holding open those doors, but I think having 2 layers would help. That said constructing such a system in existing systems seems really improbable. You would have to build it in from the beginning.

fair point

Tokyo famously has employees in white gloves who will push stragglers onto subway cars during rush hours.