A train whistle or air whistle (originally referred to as a steam trumpet) is an audible signaling device on a steam locomotive, used to warn that the train is approaching, and to communicate with rail workers.

In the context of modern and historical railroading, a train whistle is technically referred to as an air horn on diesel and electric locomotives, while on vintage steam locomotives, it is a true steam whistle. In technical and regulatory terms, particularly within the 2026 rail safety standards, these devices are part of an aircraft or train's "audible warning system." The specific patterns of sounds made by these whistles are known as train whistle signals. These signals are a standardized language used by engineers to communicate with track workers and other trains. For example, the most common signal heard at public grade crossings is the "Rule 14(l)" signal, which consists of two long blasts, one short blast, and one final long blast. In the United Kingdom and Europe, these are often simply called "warning horns." Regardless of the name, the primary purpose is safety; the sound is designed to be high-frequency and high-decibel to penetrate the ambient noise of traffic and industrial environments, ensuring that the train's presence is known long before it reaches a crossing.