Okay, so this happened at the weekend, and I'm not sure who was in the right here.
I was on a train from Paris to London. The Eurostar service doesn't stop anywhere else, so once the train has been boarded, there will be no further passengers. In my carriage, the seats were arranged so that there were three seats per "row", with one separated from the other two by the aisle. Seats alternate facing the front and back of the train, so that each seat has one opposite it, on the other side of a table. Not sure if I've described that accurately, so here's a rudimentary diagram:
[b ][ ][ ]
[a ][ ][ ]
[m ][ ][ ]
This particular carriage, there were relatively few passengers - just enough for each passenger to have their own seat and the one opposite them. I am sat where the "m" is, and there are two passengers who sat at "a" and "b". There is room for luggage either at the end of the carriage for large cases, or on a shelf above the seats for hand luggage. Alternatively, as everyone had a second seat, people had space to put purses, small bags on the seat opposite.
The journey is around 2 hours, and I had a nap about a half hour in. I woke up about twenty minutes later, and noticed that passenger "a" had put her bag on the seat opposite me. She also had a carrier bag on the seat opposite her. I leaned over and asked her if that was her bag, and the following conversation took place:
"Oh, yes. Is it bothering you?"
"Um... well. Couldn't you put it somewhere else?"
She then put the bag on the seat opposite her.
Now, was she rude for putting the bag on the seat opposite me? Technically, it was a spare seat, but there was a tacit convention that as there were only half as many passengers as seats each passenger also "owned" the seat opposite them. Was I rude? I wasn't using the seat, and was asleep for most of the journey, but it was just kind of bothering me that she'd put her bag there without asking me. I did also utilise the seat later when I went to get something from my own bag, which was on the shelf.