Thinking of the latest NY-Trenton but probably applies to other manual lapping braking systems, I'm confused as to what the "Hold" position does, as it seems to me it is the same as "Lap" - which is to keep the air pressure at the current value and keep the brake force at its current value.
Watch the gauges. In the "Lap" position airflow through the brake valve is blocked. Moving to "Hold" will fill the brake pipe again whilst keeping brake cylinder pressure.
Simply put…. LAP maintains air brake pressure and hence the brake application at its current state. HOLD (“Electric” Hold) maintains the brake application electrically but allows for the air system to release and recharge the system. As far as safety systems go like ATC and ASES, they both consider HOLD to be a “release” position so LAP only must be used to maintain the brake application during suppression actions.
