The Collaborative International Dictionary of English v.0.48:

guardrail \guard"rail`\ n.
   a railing placed alongside a stairway or road for safety.

   Syn: safety rail.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

guardrail
    n 1: a railing placed alongside a stairway or road for safety
         [syn: safety rail, guardrail]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "guardrail":
   aegis, arm guard, backstop, buffer, bulwark, bumper, contraceptive,
   copyright, crash helmet, cushion, dashboard, dodger, face mask,
   fender, finger guard, foot guard, fuse, goggles, governor, guard,
   hand guard, handrail, hard hat, helmet, insulation, interlock,
   knee guard, knuckle guard, laminated glass, life preserver,
   lifeline, lightning conductor, lightning rod, mask, mudguard,
   nose guard, pad, padding, palladium, patent, pilot, preventive,
   prophylactic, protective clothing, protective umbrella, safeguard,
   safety, safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, screen, seat belt, shield, shin guard,
   sun helmet, umbrella, windscreen, windshield