TPWS (Train Protection and Warning System) and AWS (Automatic Warning System).
AWS Alarm and Indicator
The STS AWS Alarm and Indicator Unit is part of Network Rail's Automatic Warning System and has been in use since 1983.
The Unit meets the requirements of Network Rail Group Standard GE/RT8035 and is manufactured to the requirements of British Railways Specification TEE/E/8/R/39.
Audible outputs to indicate 'Clear' or 'Caution' signals. An electronic 'chime' for Clear aspect and a continuous tone for signals at 'Caution'
A visual reminder to the train driver when the last signal was set at 'Caution'
A luminous dial for day or night viewing
A quick release bayonet connector for ease of fitting and a 5mm earth screw