Was on the train and it was pitch black outside. Thought I was sitting facing the direction of travel but turned out I wasn’t.
Even when I felt the acceleration/decelerating, I didn’t know which it was.
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It is impossible to know motion without also defining a frame of reference; it is only possible to detect changes in motion or motion relative to something else. This is the basis of the theory of relativity. Not being able to see outside the train removed any external frame of reference, thus preventing you from knowing your motion.
Similarly, acceleration and deceleration are the exact same thing, and you can only tell which is happening by an external frame of reference. Eg, the only difference is how your motion is changing relative to an outside frame of reference. Deceleration is really not a scientific term for this reason: its just acceleration.
If you are at 45 degrees latitude, you are moving west at about 700 MPH, but notice nothing as the entire surface of the planet is moving with you – you mostly notice this by seeing the Sun and Moon move in the sky. Then the Earth itself is moving around the sun 18 miles/second (30 km/s), but you feel nothing as the entire Earth is moving with you.
On a similar vein, the lack of gravity in space, such as the ISS, is not due to a lack of gravity, but rather because everything is falling exactly the same. The ISS actually has about 90% the gravity as we have at the surface, its just that the entire ISS, and its contents, is falling at exactly the same speed and acceleration* and thus there is no perceptible acceleration, resulting in a lack of gravity. There is actually a plane used by NASA called the Vomit Comet that produces the same effect for short durations – it flies up high then enters a dive that falls accelerates at the exact rate of gravity, resulting in a “lack” of gravity.
Another very interesting case you might have gotten at some point is when you are in a bus, train, or ship, and a vehicle next to you that is blocking the window starts to move. Your brain can get very confused as your eyes are seeing acceleration/motion, but your ears are not picking up acceleration. This can result in a form of motion sickness, which is tightly tied to other forms of motion sickness.
All that said, on a train you do have some outside reference that will be picked up, notably the fact that the track will not be perfectly smooth. This will result in the train bumping up and down, meaning acceleration is occurring, which you can feel. The bumpiness is directly related to how fast you are traveling along the track, and thus you can roughly figure out how fast you are traveling relative to the track by how bumpy the track is. From there, you can figure out, over time, if that rate is increasing or decreasing. You cannot, however, figure out which direction you are moving along the track, without a lot more information about the track.
* As a note, the ISS does actually have a slight difference in acceleration across it due to different distances from Earth, which is a tidal effect, but that amount is so tiny as to not matter for an object the size of the ISS.
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