This happened on a train in Malaysia years ago..Trip was Singapore to Bangok, and was a long long ride. It was night-time and I had a bunk bed and another young woman had a bed behind mine, she had her boyfriend with her. I was 22 at the time, and had limited sex experiences; mostly masturbation, and had never before seen another couple sexually involved!
I heard them whispering for a long time and figured they might try and have sex (despite the cramped conditions!) Apparently they decided the bunk was not the best place, so went off down the corridor to the bathroom. The train was pretty quiet so I figured I may as well go too.
The train was pretty primitive (no AC) I saw them both go into the men's room (oddly was pretty spacious!) I went to the adjoining womens' and while seated, noted a thin crack in the flimsy partition allowing me to see into the men's area.
They were standing up, and he had lifted her blouse, and was lifting her bra (she had small firm breasts!) By this time I was getting pretty horny, and fortunately the train had not got very busy, so no one wanted the rest room.
As I watched, I thought they were going to have sex, but he sat on the toilet, and sucked on her breasts. He had taken his pants down and she was now holding his cock, and gently rubbing it. Then he pulled her panties down, and I could see him starting to play with her pussy, still sucking her breasts. By this time I wanted to cum so bad but wanted to hold off and see what they were going to do!
I guess he knew what he was doing, as it did not take long before I saw her arch her back, and hold him with her other hand, she obviously came fast! I could not hear them due to the train noise.
He held her while she 'recovered' then started rubbing his cock again. Suddenly he shot all over the place! He missed her and just kept coming! I have never seen a guy come that much or shoot that far(before or since!)! He had left off sucking on her breasts, and was holding her hand on his cock until he finally left off!
This was too much for me to hold back I was so damned horny watching these two orgasm, so let myself go. I used one finger inside and really rubbed my clit and came hard, fortunately the train noise drowned out my moans, as I know I was pretty loud!
When I looked again, they were getting dressed, so I waited until they left, and then got ready and left myself.
I waited until they were in their bunk, and curtains drawn, then very quietly got into mine. The train had slowed, so was pretty quiet, and I heard the guy telling her that it must have been all the time they were doing foreplay that made him cum so much!
Much later, they left the bunk again. I felt I may as well enjoy this trip with voyeurism! This time he let her sit on the toilet and knelt down to play with her clit. She came fast again (I wish I could do that!) then he stood up and once again she masturbated him (she seemed to have good technique!) he came fast but not as much as he did the first time.
This time I decided I wanted to rest on the bunk and fantasize about their latest event. I managed to have two orgasms this way!
I still periodically, even after all these years, bring that couple to mind, helps me get aroused! I have seen many other events since then, many of my own, but that one couple stayed with me!
Wish I knew who the woman was, but on train trips there was no way of ever finding her.