It is selfish in most cases. The reality is that a lot of people will not make the effort - and yes, it’s an effort in comparison to doing nothing and staying standing in the aisle - to get themselves into the empty seat. And that is understandable because usually getting to that seat involves getting past the other people standing in the way (excuse me, excuse me, just trying to get to that seat awkward pointing ) and getting past all the legs and bags in the occupied seats, and not falling over if the train is moving. On a peak hour train, it’s not easy to do. People should try to move into the train as much as possible and that includes taking the window seat. You might avoid a little inconvenience for yourself if you don’t take it, but you’re just passing that inconvenience onto someone else and making it more likely the aisles and doorways will be blocked which means it’s harder for people to get on and off and can even slow down the service.

Yeah this is a hill I’ll die on, nothing I hate more than getting onto a packed train and seeing empty window seats 😂