AWS gives drivers an audible and visual indication whether a signal ahead is 'clear' (green) or not. Where a signal ahead is at red (stop) or yellow (caution), the driver receives a warning tone and distinct visual indication. The train brakes are automatically applied unless the driver cancels the warning.

TPWS is a development of the existing AWS that has been installed on the network for many years. TPWS automatically applies a train's brakes if it approaches a signal fitted with TPWS at danger too fast, or if it fails to stop at a signal at danger, or if it is travelling too fast on the approach to certain speed restrictions and buffer stops that are fitted with TPWS.

AWS & TPWS functions by sensing track mounted magnets as the train passes over them, processing this information and sending appropriate signals to the train controls and driver.

STS signals, in conjunction with the RSSB, have also developed and manufacture a range of ruggedised test equipment, specifically designed for use in workshop or depot environments:-