AWS:
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 recently, 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.
Principles of operation :
This system uses GPS for obtaining the location parameters of the train, GSM-R as a communication bearer for the track to train transmission. A Radio Block Centre (RBC) is a trackside system for transmitting field status information to the train. Vital information interface units installed along the track at signal location collect data and bring it to the central point that is the RBC. Using the information received regarding train identity, the location of the locomotive and the route data from the signaling interface, RBC shall establish the intended route of the train is clear or not. At the beginning of the journey, the locomotive is equipped with all the data regarding the signals. Loco equipment includes GPS Receiver which uses signals from GPS satellites to establish its location along the line.
AWS is part of the signaling system and warns the driver about the aspect of the next signal. These warnings are normally given 180 meters (or 200 yards) before the signal. When the AWS inductor is reached, the AWS sets a visual indicator in the driver’s cab and gives an audible indication. If the signal being approached is displaying a ‘clear’ aspect, the AWS will sound a bell and leave the visual indicator black. This lets the driver know that the next signal is showing ‘clear’ and that the AWS system is working. If the signal being approached is displaying a restrictive aspect (red, yellow or double yellow), AWS will sound a horn continuously until the driver pushes a button to acknowledge it. When the warning is acknowledged, the horn stops and the visual indicator changes to a pattern of black and yellow spokes, which persists until the next AWS inductor and reminds the driver that he/she has canceled the AWS and therefore has full responsibility for controlling the train. If the button is not pressed within six seconds, a full brake application brings the train to a halt.
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 electro-pneumatic 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 electro-pneumatic (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 (reverse) 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 reverse 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 the 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 the 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 the 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 the 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.
TPWS:
The idea behind TPWS is that, if a train approaches a stop signal showing a danger aspect at too high a speed to enable it to stop at the signal, it will be forced to stop, regardless of any action (or inaction) by the driver. The equipment is arranged as shown left.
For each signal equipped with TPWS, two pairs of electronic loops are placed between the rails, one pair at the signal itself, the other pair some 200 to 450 meters on the approach side of the signal. Each pair consists of, first an arming loop and secondly, a trigger loop. The loops are activated if the signal is showing a stop aspect.
The pair of approach loops first met by the train at 400 to 200 meters before the signal, are set between 4 and 36 meters apart. When the train passes over the arming loop, an onboard timer is switched on to detect the elapsed time while the train passes the distance between the arming loop and the trigger loop. This time period provides a speed test. If the test indicates the train is traveling too fast, a full brake application will be initiated. In case the train passes the speed test successfully at the first pair of loops but then fails to stop at the signal, the second set of loops in the signal will cause a brake application. In this case, both loops are together (see photo – right) so that, if a train passes over them, the time elapsed will be so short that the brake application will be initiated at any speed.
What TPWS Does :
TPWS has certain features which allow it to provide an additional level of safety over the existing AWS system but it has certain limitations and does not provide the absolute safety of a full Automatic Train Protection (ATP) system. What TPWS does is reduce the speed at which a train approaches a stop signal if the driver fails to get the speed of the train under control to allow him to stop at the signal. If the approach speed is too fast, TPWS will apply a full brake but the train may still overrun the signal.
Fortunately, since the train is already braking and there is usually a “cushion” of 200 yards (183 meters) between the signal and the block it is protecting, there will be a much-reduced risk of damage (human and property wise) if the train hits anything. With a possible total distance of 2000 feet (about 600 m) between the brake initiation and the block entrance, trains “hitting” the first loops at up to 120 km/h (75mph) could be stopped safely.
TPWS is also provided at many (about 3000) Permanent Speed Restrictions (PSRs) to ensure that a train does not pass through a restricted section of line (say one with a sharp curve) at too high a speed. However, there have been a number of issues related to the use of TPWS in these cases. Drivers have complained that, although they were approaching the PSR at a speed which would allow the train to run at the correct speed within the restriction, they still got stopped by the TPWS “speed trap”.
This has led to some vigorous discussions between Network Rail, the train operating companies, and the HSE.An add-on to TPWS, called TPWS+ is provided at certain signals where train speeds are above 100 mph or 160km/h.
What TPWS Does Not Do :
The safety effects of TPWS are limited by the fact that it is provided only for stop signals and that it cannot have any effect on caution signals. This means that there is a range of speeds at the higher level which will be excluded from full protection. In spite of this, it is suggested in published data that 60% of accidents due to SPADs will be prevented by the installation of TPWS at critical locations. This is achieved; it is said, at 10% of the installation costs of a full ATP system.
TPWS does not replace the existing AWS system. AWS is retained, so the driver will still get the warnings advising him of adverse signals. The TPWS equipment is designed to interface with the existing on-board wiring of trains so that it can be fitted quickly.