See the links for the background
The layout of the single compartments on our trains were like this: the compartment ran the full width of the train with doors to the outside on both sides. There were two long seats facing each other that also ran the full width of the train. Each long seat took about eight passengers and they sat facing (with knees almost touching) the passengers on the other seat.
Originally each passenger position on the long seats had a small brass plaque above it with the seat number but on most of the trains they had been removed (not sure if officially removed or if passengers had stolen the brass!). The partitions between these compartments were quite thin and it was very common for the screw holes from the plaques to have been extended through the wall to make a spy hole. Some compartments had multiple spy holes into the next carriage. Sorry for this long explanation!
On my journeys home, once I had the compartment to myself (most days as I lived near the end of the line) I would have a private self-pleasure session. I would always have a look through the spy holes and was often rewarded with something interesting to watch. I must have watched dozens of boys or men wanking (and some couples)over a couple of years. Sometimes they were unaware but often they must have known they were being watched. I was nervous about doing it myself in case the person watching might know me (more about that another time though).
One occasion sticks in my mind. Another boy about my age was in the next compartment. He was looking at a newspaper which always had a topless girl on page three. I couldn't see the picture but I knew that was what he was staring at, he didn't turn the page once and was rubbing his penis through his school trousers.
He glanced up a few times directly at my spy hole and I wasn't sure if he had seen me looking at him. After a while he really got going, and just after we left a station he stood up and pulled his trousers and underpants right down. I was impressed with his Penis and a very full bush of red pubic hair. He pumped hard for a few minutes and then shot several spurts of cum over the page three girl's picture which he had placed on the seat.
He quickly dressed, stuffed the soggy newspaper down the back of the seat and moved out of view.
The train stopped at the next station and he got out. As he passed the window of my compartment he winked and smiled, he knew all along that I had been watching! I flushed bright red after he had gone.
Naturally I also had a vigorous wank before the next station, while reliving the recent memory, but my load ended up in my handkerchief.