Automatic Warning System (AWS) is a railway safety system invented and predominantly used in the United Kingdom. It provides a train driver with an audible indication of whether the next signal they are approaching is clear or at caution. Depending on the upcoming signal state, the AWS will either produce a 'horn' sound (as a warning indication), or a 'bell' sound (as a clear indication). If the train driver fails to acknowledge a warning indication, an emergency brake application is initiated by the AWS; if the driver correctly acknowledges the warning indication, by pressing an acknowledgement button, then a visual 'sunflower' is displayed to the driver, as a reminder of the warning.

AWS is a system based on trains detecting magnetic fields. These magnetic fields are created by permanent magnets and electromagnets installed on the track. The polarity and sequence of magnetic fields detected by a train determine the type of indication given to the train driver.

A magnet, known as an AWS magnet is installed on the track center line. The magnetic field of the magnet is set based on the next signal aspect. The train detects the polarity of the magnetic field via an AWS receiver, permanently mounted under the train.

An AWS magnet is made up of a permanent magnet, and an optional electromagnet. The permanent magnet is generally uncontrollable, and always produces a constant magnetic field of unchanging polarity. A train running over the permanent magnet will deliver an AWS warning indication to the train driver.

The optional electromagnet can be used to provide the train driver with an AWS clear indication. If the train AWS detects a second magnetic field of a certain polarity after the first permanent magnet, then the AWS displays a clear indication instead of a warning indication. The train detects the electromagnet polarity after the permanent magnet polarity. This is because the optional electromagnet is always installed after the permanent magnet (in the direction of travel). The electromagnet is connected to the green signal aspect, so the driver will only receive an AWS clear indication if the signal is clear (green).

The permanent magnet always produces a south pole. If the electromagnet is energized to produce a north pole, the AWS will give the driver an AWS clear indication.

Multiple unit trains have an AWS receiver at each end. Vehicles that can operate singly (single car DMUs and locomotives) only have one; this could be either at the front or rear depending on the direction the vehicle is traveling in.

The equipment on a train consists of;