The only time I haven't had the nerve to move someone on was when I sent my 2DC on their first train journey alone. It was quite a long journey but required no train changes. I booked two window seats where they had a table between them for game playing and picnic eating.
When the train came in a very elderly and frail lady was sat in one of their seats and the reservation docket had been moved to the seat beside it. Had it been a fit and healthy person I would have politely asked them to move and pointed out the seat number printed on the moved docket but because the lady was old and frail I didn't feel I could. It was a crowded train and she clearly couldn't stand.
I knew I was the victim of emotional blackmail and that the person who saw the lady onto the train, settled her into the seat and moved the reservation docket, knew that whoever had booked the seat, wouldn't want to be seen evicting her in their favour. They couldn't know that, by good fortune (their good fortune) the seat had been booked for a big healthy teenager. DC had to sit diagonally across from each other, which made games playing and even sharing a picnic difficult. The elderly lady, like them, was on the train to the end of the line.
It still rankles