Whistle code. Train whistles are used to communicate with other railroad workers on a train or in the yard. Specific combinations of long and short whistles have specific meanings. They are used to pass instructions, as a safety signal, and to warn of impending movements of a train.

Whistle code refers to a standardized system of audio signals used primarily in maritime and rail transport to communicate intentions and safety warnings. In 2026, these remain a critical backup to radio communications. In railroading, a long blast (—) and a short blast (o) are combined into patterns: for example, two longs, a short, and a long (— — o —) is the universal signal for a train approaching a public grade crossing. A succession of short, rapid blasts always indicates an emergency or an obstruction on the tracks. In maritime navigation, whistle (or horn) codes are governed by COLREGs: one short blast means "I am altering my course to starboard," while three short blasts mean "I am operating astern propulsion" (backing up). Five or more short, rapid blasts is the "danger signal," used when one captain is unsure of another's intentions. These codes are designed to be heard over long distances and through thick fog, providing a clear, non-verbal language that prevents collisions in high-traffic zones.