A crowded train can place people physically closer than they would choose to stand beside a friend.

Physical closeness gets separated from intimacy entirely. On a full train you stand nearer to strangers than you would ever stand to someone you love, and everybody manages it by agreeing to treat the proximity as meaningless. Bodies are close and the social distance is enormous.

Which suggests how much of intimacy is chosen attention rather than nearness. If you want to feel closer to someone, the move is not proximity, which you may already have in abundance, but the small deliberate acts of attention that a crowded train specifically forbids: looking at them, asking, staying in the conversation past the first answer.