Level Crossing Barrier

Overview

A level crossing barrier is the road-facing end of a railway interlocking: a motorised boom that descends across the carriageway when a train approaches and stays down, proved and supervised, until the train has cleared. A complete crossing uses two or four barrier machines (one per road quadrant), each a self-contained unit of Boom Assembly, Drive Mechanism, and Pedestal and Machine Housing, sequenced together with Road Warning Lights and an audible Audible Warning Unit by the Control Cabinet.

The closing sequence

The cycle begins when a train passes a Strike-In Sensor on the approach, placed far enough out that road users get at least 20–30 seconds of warning before the fastest permitted train arrives. The Crossing Controller first starts the Flasher Lamp Head pairs, twin red lamps flashing alternately at 45–60 flashes per minute, hooded against phantom sunlight by Lamp Hood visors and contrasted by a retroreflective Target Board, together with the Electronic Sounder. After a fixed pre-warning of a few seconds, the booms descend over 6–10 seconds. On four-quadrant crossings the exit booms lower a few seconds after the entrance booms, so a vehicle caught on the crossing can still drive out.

While a train occupies the Island Circuit spanning the roadway, the booms are locked down. Only when Wheel Detector heads have counted every axle out of the section does the controller raise the barriers and extinguish the lights. Full-closure crossings add an Obstacle Detection Radar that scans the enclosed road area; the protecting railway signal clears only after the scan confirms the crossing is empty.

Drive mechanism

The boom itself is nearly weightless to the motor. Cast Counterweight blocks behind the pivot balance the Boom Arm to near-neutral torque, so the Drive Motor, typically only 0.25–0.75 kW, works mainly against friction and wind. Torque passes through a self-locking Worm Gearbox and a Crank Linkage whose geometry goes over-centre at both ends of travel: in the raised and lowered positions the crank is past its dead point, so no back-load from the boom can rotate the mechanism. The worm gear holds position when power is lost, and slightly counterweight-heavy balancing is usually chosen so a totally failed machine can be lowered by gravity or wound by the interlocked Hand Crank.

Cam-driven Limit Switch contacts prove the true boom position to the controller, the logic never assumes; it checks. A Friction Clutch lets the drive slip if the boom meets an obstruction, and a Breakaway Coupling at the arm root sacrifices the boom rather than the machine when a road vehicle strikes it, the most common failure event in service.

Fail-safe control

Crossing logic is vital in the signalling sense: every step must be proved before the next is allowed, and any failure must leave the crossing in its protective state. The Flasher Driver proves current in each lamp; a failed lamp is reported and, beyond a threshold, the crossing is treated as degraded. The Limit Switch proving of "booms down" is a precondition for the railway signal in rear to clear. Float-charged batteries in the Backup Battery Bank keep the whole installation running for 12 hours or more after a mains failure, and an Event Recorder time-stamps every activation, lamp proof, and boom movement, the first artefact examined after any crossing incident.

Structure

Each machine bolts to a concrete foundation through a Foundation Frame sized for the overturning moment of a 9 m boom in design wind. The lockable Machine Housing encloses the drive; the Pivot Shaft passes through sealed Ball Bearing journals to the boom and counterweights outside. Along the arm, Boom Lamp lamps mark its length to road traffic, the tip lamp flashing, the inner lamps steady, and on pedestrian routes a hanging Boom Skirt discourages ducking under a lowered boom.