On a long train ride you run out of distractions and start running into yourself.

Distraction has a bottom, and a long ride finds it. Once the phone is flat and the book has worn out, there is nothing left between you and your own thinking, and whatever you have been postponing turns up with time to spare. That is uncomfortable for a while and then it is not. Most people never get far enough into the boredom to reach the other side.

If you have a long ride coming, do not plan it perfectly. Leave a stretch deliberately unfilled and let the quiet get awkward. Keep something to write with, because past the first hour there is usually something worth keeping.