The mysterious black knob
The OP was riding on a train one day and discovered a black knob popping out of one of the armrests. Instead of fiddling around with it like any person would, they decided to upload a picture and ask the internet what it was. Since they’re so common, it took no time for someone to deliver the correct answer.
This black knob controls your seat’s footrest. If you want to lift your legs up a few inches, just pull the knob out, and the footrest will lift from under your feet. We wish we had known this a lot sooner. We thought trains were made to be uncomfortable, but we were dead wrong.