red flag

A red flag is a cloth signal, typically square or rectangular, mounted on a short staff or handle, used by railway personnel to communicate an emergency stop or warning to a train driver or locomotive crew. Historically, it was the primary visual signal available before electric and pneumatic systems became standard. The guard or signalman would wave or display the flag to indicate that the train must not proceed, or must halt immediately if already moving.

Red flags were essential on railways from the mid-nineteenth century through much of the twentieth century, particularly on branch lines and during shunting operations where line-of-sight communication between crew members was critical. The flag's bright red color was chosen for high visibility in daylight and across distances of 200 to 500 metres depending on weather and background. Many railways used flags of different colors: red for stop, yellow or amber for caution, and green or white for proceed. The flag was often kept in a leather pouch or holder attached to the guard's uniform or hung near the brake van.

Decline and survival

Electric and pneumatic signaling systems, along with radio communication, progressively replaced flag signals from the 1950s onward. However, red flags remain in use on heritage railways, during engineering possessions when normal signaling is unavailable, and in some countries' rail operations as a backup method. Modern rail workers may carry a folding red flag made of synthetic fabric for use during track work or emergency situations where radio contact is lost or unreliable.

The term red flag has acquired a broader figurative meaning in industrial and general vocabulary, referring to any warning sign that something requires immediate attention or suggests danger. In railway operations today, the phrase survives in training contexts and in written procedures that specify conditions under which flag signals must be deployed, even though their practical use has contracted sharply.