Common courtesy
I know the train will be full. I know you want to get a seat. But it won't be that full. Not yet at this station. There will be room for everyone. And I too would like to sit on the window side.
Yet, I don't rush in. I let you go first as it seems you are in a hurry. Trust me, the train won't be any earlier despite you rushing in. And it won't leave you at the station. At least you could let people stepping off the train do so first.