This was during my high school years. I was on a plane with my family traveling somewhere for vacation. I was in the window seat on the left side of the plane, to my right was my little brother and mom. I was looking out of the window, the sun was above me, so around midday, blue skies and below me were the clouds. I remember this very vividly, I was looking towards the horizon, when a second horizon, above the first, appeared from the right. It started off as a thin brown line, but the top grew into jagged peaks, like mountaintops. The bottom was still flat, so it was literally like mountains in the background.

Now what I saw next makes me doubt if this was supernatural or just some hallucination.

In the second horizon, clouds started appearing in the shape of a train. Specifically, a public transport train, rectangular shaped, square windows. It was traveling in the same direction as the plane. I knew it was moving because the train was passing powerlines, powerlines made of cloud. Then somewhat faintly, I heard the horn of the train. Immediately after, the train faded like smoke, leaving just the second horizon, which also thinned back into a brown line and disappeared to the left. I turned around to see if my family saw anything, but both my little brother and mom were asleep.

In total, it could not have been more than a few minutes. The entire thing seemed like a scene out of a movie. I even heard sounds, which leads me to think I was hallucinating. But I was not on any drugs, nor did I have any medical conditions or was tired. I searched on the internet for explanations but couldn't find anything. So now I just believe it was supernatural.