Newer automated systems have the benefit of foresight, and usually contain walkways the length of a tunnel with evacuation cross passages.
In a legacy system like New York or London where these facilities don't exist and introducing them would be very disruptive, if not completely impossible, you probably need at least one person on a train to assist with evacuation.
In a legacy system like New York or London where these facilities don't exist and introducing them would be very disruptive, if not completely impossible, you probably need at least one person on a train to assist with evacuation.