I was travelling from London to Sheffield late one night by train, which left Kings Cross at 10.15 pm. I hurried to the station where I took my seat and was a little disappointed to see it was some old rolling stock. Instead of the ultra modern 125 locomotive it was a box standard diesel train with the corridor type coaches.

Thinking about it though, it wasn't too bad for it meant that once the train started, you could close the section door and snuggle down to some sleep whilst the train sped through the night. I found my appointed coach and section again I was dismayed to see two other people sharing my section of the coach. I young man in his early thirties and a young woman I estimated to be no more than twenty-five or so.

I suppose you the readers are wondering what this couple looked like; well I will attempt to do their descriptions justice. He was around five foot eight tall, fairly slim build with what can best described as a rugged appearance. I could quite easily see him turning more than a few ladies heads. The girl was a few inches taller than her male friend; long blonde hair cascaded down to shoulder length and her petite figure was crowned with a generous bust line, which hinted a deep cleavage as she sat with the top three buttons of her blouse open.

I did not pay any more attention to them as I settled into my seat opposite the pair, soon the whistle sounded and the train started to inch its way out of the station heading north.

After about five or six minutes, the girl giggled and disturbed my attempts at dropping off to sleep, I thought bloody great, if they are going to be teasing each other all the way, I would never get any sleep. Through half open eyes I saw the guy look in my direction before whispering something to the girl, she immediately looking in my direction and nodded.

I was intrigued, was my flies undone or had they something else in mind, I soon discovered as I kept my eyes almost closed and waited. The girl slid down in the seat, somewhat and opened her legs as wide as she could. So that was their little game, they intended flashing at me, hoping I would not see them but excited by the thought of being caught.

Through half closed eyes I saw she was wearing white panties with a developing wet spot right over her cunt making that section almost see through. What was I to do, I could keep quiet and see what developed or I could speak up and either frighten them off or encourage them further.

I decided to bide my time to start with and tried desperately not to show my interest although I was sure if they looked properly they must see my developing erection. After a little while of exposing her panty cover cunt to this stranger the guy reached over between her legs and pulled the crotch of her panties aside. He then slipped his index finger deep inside her wet cunt; this had an immediate effect upon her breathing, as her breaths became short ragged affairs.

To be honest, it was also having a profound effect on my cock, as it was now threatening to rip its way out of my ever-tightening pants. After several minutes of his thrusting he pulled his finger from her cunt; much to her disgust and he proceeded to suck the finger clean of her cunt juice.

She leaned over to him and whispered something to him, again the pair looked in unison towards me and he almost blushed, but then he nodded and made to stand up. Suddenly he undid his trousers and dropped them to his ankles, his underpants were quickly travelling in the same direction and he stood side on to me and began to wank his young firm cock. The girl leaned forward at this and suddenly engulfed his cock in her mouth.

I really could not stand by and witness much more of this without yanking out my own cock and at least wanking it, although the thought of offering her another cock to suck was severely tempting.

I suddenly sat bolt upright and demanded to know what the fuck was going on. In feigned shock horror the couple began apologising profusely, claiming their passions got the better of them and they hoped I would not report this to the police or anything. Never one to miss an opportunity, I hinted I could be persuaded to forget the incidents I had witnessed but I would want to see them both stark naked as an incentive.

The couple looked at each other before nodding and in a flash both were stood before me stark naked. I asked them their names and discovered that Jill was in fact John's sister. I asked them if their parents were aware of their incestuous relationship, I was shocked when Jill explained that her mother and father had introduced them to the pleasures of sex when they were both sixteen, although she had the extra pleasure of watching many threesomes between her brother and parents whilst she waited five years to reach sixteen. This shocked John slightly as it was obviously news to him that his sister had been aware of his special relationship with his parents.

I asked them their current ages and john answered I am thirty-two and my sister is twenty-seven. So you have indulged in sex between you for eleven years, I offered. Both nodded as they sat either side of me. Jill then suggested that I get comfortable by removing my clothes as she pointed to my bulge saying that looks a little uncomfortable.

She continued to tell me of her voyeur experiences over her brother and her parents. "I remember one wonderful night when Daddy told John he was going to show him some real fun called a stacker stack. She had almost laughed at John's puzzled look but had managed to refrain from doing so, not wanting to give her presence away"

She continued by describing the stacker stack as she called it; She reminded john that it was when his mother knelt between his legs and began sucking his beautiful cock whilst his dad had lubricated John anal region before slipping daddy's large and long cock up his arse. When he was fully home up John's arse, mummy stopped her oral ministrations and turned round taking John's cock up her own arse. From then on John was like a shuttlecock between fucking his mother's arse and having his own arse reamed by a monster cock. As she described this I noticed John's cock jerk a couple of times, obviously as he recalled the experience.