aspect
The visual indication of railway signal as displayed to the driver. With three-aspect colour light signals this would be red, yellow or green, and on four-aspect signals, double-yellow also; a two-aspect signal displays red or green.
aspect: what a signal tells the driver to do
An aspect is the displayed state of a railway signal at any given moment, conveyed through colour, shape, or light pattern to inform the driver of track conditions and speed authority ahead. On a colour light signal, the aspect is simply the colour currently illuminated: red means stop, yellow means caution, green means proceed. The driver reads this aspect and adjusts speed and braking accordingly, making it the primary command mechanism between signalling infrastructure and locomotive operation.
The number of aspects a signal can display determines how much information it can convey. A two-aspect signal shows only red or green, typical on simple branch lines or rural sections where speed restrictions are minimal. Three-aspect signals add yellow (cautionary yellow), giving drivers advance warning of a red signal ahead. Four-aspect signals introduce double-yellow, a second warning aspect that appears two signal blocks ahead of red, allowing faster speeds on main lines while still providing adequate braking distance at typical line speeds of 100 mph or more.
Semaphore and searchlight variants
Before colour light signals dominated, semaphore signals conveyed aspect through arm position: horizontal meant stop, 45 degrees meant caution, and lowered at 45 degrees meant clear. Searchlight signals, an intermediate technology still found on older routes, use a single white light source directed through colour filters and lenses to show aspects; their mechanical complexity made them less reliable than modern colour lights, which use discrete red, yellow, and green lamps that fail independently and safely.
Aspect is not the same as indication. The aspect is what the signal shows right now; the indication is what it means in context, determined by the signalling system's rules and the track layout. A yellow aspect means caution, but whether the driver must reduce to 15 mph or 60 mph depends on the system's speed code and the nature of the restriction ahead. Multiple signals with the same aspect can carry different indications depending on their location in the route.
Signal failures are classified by their aspect behaviour. A stuck red signal causes delay but no danger; a stuck green signal is a safety hazard and triggers manual intervention. Colour light signals are designed to fail dark (no light), though lamp failure detection systems now illuminate a lunar white light to indicate malfunction rather than leaving drivers guessing. The reliability of aspect display depends on maintaining lamp cleanliness, filter integrity, and electrical contact in exposed trackside conditions.