My husband and I were travelling across the continent by rail and had planned one overnight in a sleeping car.
We were dead tired after a day on our feet and entered our compartment and were pleased to find we were the only occupants. We slept on pull-down bunks on the forward side of the train so that we wouldn't roll off in any sudden braking.
Hours after we were asleep, the train became crowded and another couple entered and quietly went to sleep on the facing bunks.
Something awoke me in the moonlight and I looked over to see a woman in a bed only a few feet away from me masturbating languidly as she watched me and my husband sleep. When her eyes met mine she was startled for a moment, but then smiled and continued.
I watched her just long enough to become tingling down there myself and furtively moved a hand between my legs. She noticed and pushed aside her own cover boldly. The ambient light was sufficient for me to see her fingers move along her slit and occasionally dip inside herself for more wetness.
I removed my own sheet and opened my legs as well and we both masturbated as we watched each other's hands and eyes. As she began her orgasm mine rushed to the fore and we finished cumming together.
She smiled again, as did I, and we drifted off to sleep.
The next morning we left the train before the other couple was fully awake. We have no other overnight rail trips planned, but I'm hoping to add one now.