Railway Color-Light Signal Product

Overview

A color-light signal is the trackside half of a railway's movement authority system: the interlocking decides which aspect a train may receive, and the signal displays it as a colored light visible far enough ahead for the train to brake. A typical main signal stacks three or four light units in a Signal Head on a Mast Assembly beside the track, with a matte Background Plate behind the lights so a lit aspect reads clearly against bright sky. Red means stop; yellow warns the next signal is at red; green clears the line; a second yellow extends the warning chain on high-speed routes. A Route Indicator above the head adds a white character telling the driver which diverging route has been set.

Light units and optics

Modern installations use LED light units in place of filament lamps. Each LED Light Unit packages a colored LED Array with a LED Driver Board that regulates current for full daytime output and a dimmed night level. The wavelengths are tightly controlled — signal red near 630 nm, yellow near 590 nm, green near 505 nm — because drivers must distinguish aspects without ambiguity in fog, rain, and low sun. LED service life of around 100,000 hours removes the periodic relamping that filament signals required.

The optical front end shapes where that light goes. In each Lens and Hood Assembly assembly, a 200 mm Outer Lens collimates the beam into a 1–3 degree cone aimed at the driver's eye position several hundred metres out, while an inner Spreadlight Lens bleeds a controlled fraction downward and sideways so the aspect stays visible in the final approach and on curved sightlines. A deep Signal Hood over each lens solves the classic phantom-aspect problem: without it, low sun entering the lens can reflect off the internal optics and make an unlit aspect appear lit. The Alignment Mount lets installers aim the beam axis at the sighting point during commissioning, checked from track level with a sighting telescope.

Proving and fail-safety

Railway signalling is built on the principle that any failure must produce a more restrictive indication, and the lamp circuit is where this gets concrete. The Lamp Proving Unit in the Control Electronics continuously measures current in the lit aspect. If the light fails, the proving relay drops, and the interlocking responds by holding the signal in rear at a more restrictive aspect — a dark signal is treated as a stop signal, never as permission. Because legacy proving circuits expect a filament's current signature, each LED unit includes a Current Monitor that presents a lamp-like load and opens the circuit cleanly when enough LEDs in the array have failed.

Aspect selection itself runs through vital Relay circuits or the electronic Aspect Controller, arranged so only one aspect can be energised at a time and so a flashing aspect proves both its on and off phases. Every tail-cable circuit enters through a Surge Arrester, since signals live metres from 25 kV traction feeders and attract lightning on exposed masts.

Structure and installation

The Signal Mast is hot-dip galvanized tubular steel, typically 4–6 m tall, bolted through a Base Flange to a concrete foundation with levelling nuts that set the head plumb. A hooped Access Ladder and a small Maintenance Platform give maintainers access to the Rear Access Door at the rear of the head, so light units can be swapped without dismounting anything. The retroreflective Identification Plate carries the signal number used in all dispatcher–driver communication.

Power and control arrive through the Cabling and Termination: steel-wire armoured Tail Cable runs from the location case, terminated on linked Terminal Block rows at the mast base so each aspect circuit can be isolated and meter-tested independently, with Cable Gland entries keeping the IP65 sealing intact. The whole signal draws only tens of watts, but its proving circuits participate in the interlocking's safety case — which is why even the humble Lens Gasket matters: condensation inside the optical chamber can dim or distort an aspect that a driver is reading at line speed.