This article is based on information provided by Prakash Tendulkar and his friends in the Mumbai suburban systems of CR and WR and IRFCA members.
AWS or the Auxiliary Warning System is a system of providing some advance notification of upcoming signal aspects to the motorman via a display panel in the driving cab of the EMU. The advance notification is done through trackside electromagnets that trigger relays in the passing EMU cabs. This was originally introduced by Western Railway for its Mumbai EMUs in the early 1980s. Central Railway introduced AWS somewhat later. Initially there were many teething problems with technology, including problems of pilferage of the trackside equipment, but these were mostly resolved by the 1990s. CR's implementation of AWS was initially incompatible with WR's, which meant that each railway's EMUs could run on the other's tracks but without the benefit of the AWS system. More reecently, the systems have been made compatible and the two railway's EMUs can run on each other's tracks at speeds up to 100km/h taking full advantage of AWS.
While there are still some differences between the two railways' AWS systems, the following description, which is based on the WR AWS, should give an idea of how the system works.
At a high level, the AWS can be thought of as having three different principal states for responding to different signal aspects:
The AWS panel in the driving cab has an alarm buzzer, a vigilance button, and can show indicator lights in red, yellow, or blue. When the buzzer alarm goes off, the motorman must press the vigilance button with 4 seconds, otherwise the brakes (either electropneumatic or emergency pneumatic) will be applied and the motorman will not be able to release them until the rake comes to a complete halt. There is also interlocking between the AWS and the electropneumatic (EP) brakes such that when the rake is in motion and the EP switch is on, the motorman cannot place the master controller in neutral and set the forward/reverse switch (reverser) in neutral. Attempting to do so results in an immediate application of the emergency brakes. (However, if the EP switch is off, the master controller and the reverser can be set in neutral.)
The following possible conditions arise with AWS and different signal aspects:
Both WR and CR are continuously enhancing their systems, and AWS is undergoing upgrades as well. Most recently, the introduction of the Train Management System (TMS) has resulted in some changes integrating AWS into TMS.
AWS display unit in driving cab.
Photograph by Prakash Tendulkar, © 2001.