Train Nomads

Although I've yet to get on a train carriage that was full, they are sufficiently full to breach your personal privacy threshold. The layout is three seats, then the aisle, then two seats, like this:

--- --

And they tend to fill up like this:

O-- --
O-- -O
O-O -O
O-O OO

At this point the carriage is 80% full and the three-width seats all have the middle seat empty. No-one has a fellow passenger on both sides. This is the privacy threshold and this is when the train nomads appear. The train pulls out of the station and a few seconds later people starting entering the carriage through the vestibule, passing through and out the other side. At first I thought they were people who had jumped on at the last moment, and were working their way to the empty section, but there are too many of them for that and they pass in both directions. They are The Wanderers. They roam the train looking for a row with two empty seats, preferring their journey within a journey to having to sit between two strangers.

And thanks to them, there are more empty seats.