But things can go wrong.
I can't get these images out of my head. On a screen I see a mother with a baby stroller, last in line to board a subway car. She pushes the baby towards the door, ignoring the warning tone. The door closes, pinching a small part of the stroller. The train moves, pulling the stroller along the platform. The desperate mother won't let go, she's dragged along the platform, her attempts to release her child are futile. They speed towards a barrier at the end of the platform. People look on, alarmed, helpless.