They do not want you to and actively try to stop you by closing some platform doors, but it’s still possible.

The reason for it is that the platforms are so narrow that allowing boarding passengers to wait there would make it slower for passengers to disembark.

One or two people who figure out the trick doesn’t pose that issue, but you’ll never hear official approval of it.

I do it every other time or so depending how resourceful I’m feeling. It used to be way easier when there was an arrivals board in the waiting area!