A holding brake (often called a "parking brake" or "spring-applied brake") is a safety-critical mechanical system designed to keep a train stationary when it is not in use or when the primary air brakes are exhausted. Unlike the service brakes used to slow a moving train, which typically require active air pressure to apply the brake shoes, many holding brakes are "fail-safe." This means they use heavy-duty internal springs to physically clamp the brake pads against the wheels or discs. To release the brake, the locomotive must apply air or hydraulic pressure to compress the spring and pull the pads away. This ensures that if a train loses all power or air pressure while parked on a grade, the holding brakes will automatically engage (or remain engaged), preventing a dangerous runaway scenario. In modern electric and diesel-electric trains, these are often managed electronically but still rely on the physical force of a spring to ensure reliability during a total power failure.