Limitations
TPWS has no ability to regulate speed after a train passes a signal at "stop" in accordance with Stop and Proceed rules. However there are strict rules governing drivers' actions and train speed when passing signals at "danger" with authority and the use of TPWS on these occasions. Thus TPWS would not have helped prevent the Glenbrook train disaster.
There are several reasons why a driver would be required to pass a signal at danger with authority, such as track circuit failure. The driver will have been advised by the signaller to pass the signal at danger, proceeding with caution and being prepared to stop short of any obstruction and to obey all other signals. The signaller would expect the driver to know the requirement of pressing the "Trainstop Override" button on the TPWS panel to pass the signal without triggering a TPWS brake demand. A signaller is not allowed to inform the driver to operate any TPWS override, only that TPWS may be present at the signal that is required to be passed at Danger.
Due to the varying conditions under which a train may need to pass a signal at danger, it is seen as best to leave the appropriate speed to the driver's discretion, rather than have a fixed speed. The appropriate speed for a heavy freight train on a curved line during heavy rain will be much lower than for a fast braking passenger train on a straight line in clear and dry conditions.
Whilst critics claim TPWS is a cheap solution and putting lives at risk compared to fitting ATP, there have been very few fatalities in modern times (since the fitting of AWS) that would have been prevented had ATP been fitted but would still have occurred despite TPWS. The Southall rail crash would not have been prevented by TPWS yet could have been prevented by ATP (ironically fitted but not in use), yet would almost certainly have been prevented had the AWS been working. A combination of TPWS and AWS is most weak against accidents like that at Purley, where a driver repeatedly cancelled the AWS warning without applying the brakes, passing the danger signal at high speed. In this particular case though, the lower speed of the train and the very effective brakes of the EMU would have meant TPWS would likely have been effective anyway. Supporters of TPWS claim that even where it could not prevent accidents due to SPADs, it would likely reduce the impact and reduce or eliminate fatalities by at least slowing the train down but, in practice, it is likely that in these cases the driver would have already applied the emergency brakes well before the overspeed sensor.
Read more about this topic: Train Protection & Warning System
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