Mumbai Suburban - Auxiliary Warning System

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:

Signal aspect is Red : Speed is limited to 15km/h.
Signal aspect is Yellow : Speed is limited to 38km/h.
Signal aspect is Green or Double Yellow : Speed is limited to 70km/h.

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:

Signal blank, in Auto mode: AWS shows steady red indication. Motorman must stop 100m before signal, and press the vigilance button within 4 seconds when the buzzer alarm sounds. After waiting for the signal to change aspect, if the signal remains blank, the motorman can proceed cautiously at 15km/h up to the next signal.
Signal blank, in Manual mode: AWS shows a fast flashing red indication. Motorman must stop 100m before signal, and press the vigilance button within 4 seconds when the buzzer alarm sounds. He can only proceed past the signal after receiving authority to proceed from the section controller (written or telephonic).
Signal Red, in Auto mode (permissive red): AWS shows steady red indication. Motorman must stop 100m before signal, and press the vigilance button within 4 seconds when the buzzer alarm sounds. After waiting for the signal to change aspect, if the signal remains blank, the motorman can proceed cautiously at 15km/h up to the next signal.
Signal Red, in Manual mode (absolute red): Motorman must stop 100m before signal, and press the vigilance button within 4 seconds when the buzzer alarm sounds. He can only proceed past the signal after receiving authority to proceed from the section controller (written or telephonic).
Signal Yellow: AWS shows steady yellow or flashing yellow indication depending on whether the next signal in the route is connected with the same circuit as the present one. The yellow signal should not be approached faster than 60km/h when 700m before the signal. Speed must be reduced to 38km/h when passing the signal. The motorman must press the vigilance button within 4 seconds when the buzzer alarm sounds. Further speed restrictions may be in effect if the route is set for a diverting line.
Signal Green or Double Yellow: AWS shows a steady blue indication that shuts off after 4 seconds. The motorman does not have to carry out any specific actions, except to regulate the speed to below 70km/h.

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.