This is one of my enduring memories of NYC as I saw this precisely play out first hand. A tiny woman with a massive stroller arrived at the base of the stairs and the guy walking in front of her glanced back then gave the slightest nod before reaching down and grabbing the front axle. She overhead pressed the stroller handles, they climbed a couple of flights and at the top of the stairs he put his end down. Not a word exchanged, not even a thanks, they just went their separate ways. It was so smooth, almost choreographed, I don't believe the woman even broke stride.