In Britain it is considered customary to occupy the aisle seat only and use the window seat as a handy baggage store. It is also considered good practice to make sure that your headphones and music volume are carefully adjusted so as to ensure that the occupants of seats immediately in your vicinity can easily enjoy the tsshhh tsshhh tsshhh of the, preferably repetitive, treble notes.
If you find that you are able to occupy a table seat the usual rules of aisle vs. window seat and using the seat to conveniently store your baggage apply but in addition don't forget to allow your neighbour opposite to admire your footwear by extending your legs as much as possible. Don't deny them the opportunity to admire your phone, laptop and tablet either by selfishly keeping them towards you but instead be sure to spread.
You may find that a seat has been reserved; this reservation guarantees the customer the absolute right to the seat even if either they don't turn up or the train is not within the zone of the reservation; seats with reservation tickets must be left empty at all times.
Also, don't forget to leave the vestibule luggage racks empty and wheel your suitcases down the aisle to provide an entertaining high jump for any passengers trying to walk down the train.
Where a train has a bicycle compartment, this is for the exclusive use of those passengers who have to get off the train first and need to be by the door five miles before their station so don't expect them to move elsewhere allowing the seat to fold up and your bicycle to be safely stowed. The other passengers love having to squeeze by your bicycle too.
In the event that there is no seating left (because is is rightly occupied by luggage) it is essential that you cram into the vestibules and in no circumstances should you stand inside the car or make way for those who would like to.
On the subject of food, it is traditional to keep your fellow travelers enticed by wafting as much of the aroma of your burger or sandwiches in their direction as is possible. Don't forget to leave the wrappers behind the tray table either - that way even those people getting on after you have long since left will get to appreciate your dining choices.
I am sure that the same etiquette applies elsewhere and so I hope this helps.