There is an intimacy to riding the subway/metro, in part because of the culture and sense of pace. When walking, I am alone. At most I'll nod at a passer by. I can't speak to Korean culture, but in DC, even during commutes, some people don't expend the energy to wear a mask on the train. Often, people are still waking up or too tired from a long day to pretend.
People reveal themselves through moments, such as the ones Jared described, but there is no expectation that I have to reciprocate. I can observe. This helps me settle and reflect.
The speed of the train has a similar effect as the car on my family's mini-dachshund, it relaxes her. I am not focused on my surroundings, rather myself and my immediate and transient neighbors.