safety chain

from The Century Dictionary.

• noun On a railway, an extra chain or coupling attached to a platform or other part of a car to prevent it from being detached in case of accident to the main coupling; a check-chain of a car-truck; a safety-link.
• noun A short chain placed between a car-body and the truck to prevent the latter from swinging at right angles to the body in case it leaves the rails while the car is moving; a check-chain.
• noun A light chain designed to be attached to a key-ring or watch to prevent it from being stolen from the wearer's pocket.

from the GNU version of the Collaborative International Dictionary of English.

• (Railroads) A normally slack chain for preventing excessive movement between a truck and a car body in sluing.
• An auxiliary watch chain, secured to the clothes, usually out of sight, to prevent stealing of the watch.
• A chain of sheet metal links with an elongated hole through each broad end, made up by doubling the first link on itself, slipping the next link through and doubling, and so on.
• (Arch.) a discharging arch.
• a belt made of some buoyant material, or which is capable of being inflated, so as to enable a person to float in water; a life preserver.
• a buoy to enable a person to float in water; a safety belt.
• (Mach.) a cage for an elevator or mine lift, having appliances to prevent it from dropping if the lifting rope should break.
• (Mining) See under Lamp.
• a match which can be ignited only on a surface specially prepared for the purpose.
• a pin made in the form of a clasp, with a guard covering its point so that it will not prick the wearer.
• See Fusible plug, under Fusible.
• See Switch.
• (Football) the act or result of a player's touching to the ground behind his own goal line a ball which received its last impulse from a man on his own side; -- distinguished from touchback. See Touchdown. Same as safety
• (Chem.) a tube to prevent explosion, or to control delivery of gases by an automatic valvular connection with the outer air; especially, a bent funnel tube with bulbs for adding those reagents which produce unpleasant fumes or violent effervescence.
• a valve which is held shut by a spring or weight and opens automatically to permit the escape of steam, or confined gas, water, etc., from a boiler, or other vessel, when the pressure becomes too great for safety; also, sometimes, a similar valve opening inward to admit air to a vessel in which the pressure is less than that of the atmosphere, to prevent collapse.