starting signal

A starting signal is a railway signal that authorizes a train to begin moving from a platform, siding, or other stationary position. Unlike approach signals that warn of conditions ahead, the starting signal is the final clearance: when it displays its permissive aspect, the train driver receives explicit authority to release brakes and move forward. On modern railways, this is almost always a colour-light signal displaying green, though older mechanical semaphore signals with arms angled downward served the same function.

Starting signals sit at the exit point of a station platform or at the limit of the departure line. They protect against conflicts with other train movements in the track ahead: a signal will not clear unless the section of railway beyond it is known to be unoccupied and safe. The signal works in tandem with interlocking apparatus, whether mechanical lever frames or solid-state electronic systems, which prevents conflicting routes being set simultaneously. If a train is occupying the track ahead, or if another route has already been cleared in a conflicting direction, the starting signal will remain at red (or danger in semaphore terms).

The distinction between starting signals and running signals (those that govern movement between stations) is operational rather than mechanical. A signal post may display different aspects depending on context: it functions as a starting signal when a train is stationary at a platform, and as a running signal once that train is in motion. Drivers must recognize the signal's location and purpose; misinterpreting a signal can lead to unauthorized movement or collision.

Failures in starting signals most often arise from lamp burnout in colour-light installations, particularly the green lens, or from track circuit defects that prevent the signal from clearing because the system does not recognize the line ahead as free. In mechanical systems, sticky or corroded arms, or failure of the locking mechanism controlling the signal lever, prevent clearance. Modern signalling practice requires regular testing of signal aspects and circuit integrity to catch these faults before they delay traffic.

The term "starting signal" has remained in use across different railway systems despite the variety of signalling designs, because the functional role is universal: it marks the point where authority to move begins. Some railways use different terminology, such as "departure signal" or "platform signal," but the principle is unchanged. The signal's placement and logic are defined by the railway's operating rules and the design of its interlocking system.