You've walked onto a train that is less than half full. How do you decide where to sit? There is an understood rule in restrooms, at least among men, that you don't take the spot immediately next to one that is occupied. More accurately, some guys would say you take the spot the farthest distance away, and then only begin taking the ones in the middle in a sort of binary "keep splitting the distance so you're equal distance away" sort of way.
That's a little excessive when it comes to a train with 50 rows of seats, but I did notice that the spread comes out relatively similar. There's someone in a row, and then three empty rows, and then someone in the next row. I don't know about you, but I gravitate toward the one in the middle. I realize that all the empty rows are probably going to get filled up anyway, but when it's my pick, I like space on both sides. Since I work on the laptop I tend to prop my knees up, normally against the seat in front of me. If I sit right behind somebody I'd find it rude to shove against their seat like that. I don't sit directly in front of people, if I can help it, for the same reason - I don't want to be the one crunching the other guy's legs out from under him.
Why is it that the person who always sits down behind me seems like they need to bang on my seat for 10 minutes before they get comfortable? What in the world are they doing?