See earlier stories to understand the single compartment trains and the spy holes between the compartments
While travelling on the single compartment trains as a teenager, I had lots of fun experiences. Many of these were watching sexual activities in neighbouring compartments through the spy holes.
One evening it was dark outside and I watched a young couple, not much older than I, taking advantage of an otherwise empty compartment and making out during the travel between stations. At first they were just kissing and he had his hand inside her blouse.
After one stop I was very happy when she undid her blouse and revealed perfect (to my adolescent mind at least) young breasts, she wasn't wearing a bra. After she let him kiss her breasts very sensuously, she reached down and undid his fly. She reached inside and pulled out a thin but very long uncircumcised cock. She started to pull his long foreskin up and down very slowly. We were arriving at the next station so they quickly covered up. He had some trouble getting that long and hard penis back into his trousers in time.
The train quickly took off again and they wasted no time getting back to where they were. This time she started to masturbate him more quickly and it was obvious that she was quite expert with her hand movements. I was following the timing of her strokes exactly and getting quite close to my own orgasm. I was planning on holding back so that I could come at the same time as this lucky guy.
Again the train pulled into a station so we all covered up but a few minutes later we were underway and they were at it again. I watched her slowly caress the head of his penis. After a few minutes she sat up and I could see the head of his penis exposed with the foreskin retracted. It was dark red and slimey with his precum. She then went to work again with her hand slowly advancing and retracting his quite tight foreskin over the head, gradually picking up her pace until she was going quite fast. I could see from his face that he was getting close to orgasm. He then put his hand upon hers to slow her movements down a little and to make her perform longer strokes. Again I was copying the movements and manipulating my own foreskin up and down in time with his. Suddenly he threw his head back, grunted and out shot his sperm. I orgasmed again immediately as he did this.
He cleaned himself up with a hankerchief and then kissed her breasts tenderly for a few minutes before they both dressed up. She never touched herself, and he didn't touch her apart from her breasts.
At the next station they go out. They glanced into my carriage and I blushed intensely red but I don't think they noticed. I'm pretty sure they didn't know I had been watching.
I have relived this experience many many times in my head and always enjoy a great orgasm, and sometimes two.