(19-03-2010, 04:20 PM)Jerry1237 Wrote: I think the answer is that it is "out of position" to differentiate to the driver that they are traversing from an unfitted line to a AWS fitted one but the standard certainly doesn't make why clear. Please note, the AWS is suppressed for train movements that continue along the unfitted line. The whole AWS in advance of the signal is peculiar to an movement from a through unfitted line onto a fitted one!
Jerry
The issue is that for a train carrying on along the UNFITTED line it does NOT want to get a one-off AWS warning as it approaches a signal. Therefore it is logically not possible to put AWS prior to the signal as can't necessarily know where that train is going. By placing after the signal then the magnet can be supressed, so that a train on the non-fitted route will not receive any AWS warning; a train going onto the fitted line gets the AWS warning to prepare the driver for all the subsequent ones that they will be getting once they join the fitted route. Remember that AWS is all about giving a warning at "the distant"; if a train joins the fitted line it could potentially encounter a signal at red without having had prior warning (and therefore more likely to SPAD). The fact that it has to be after the previous signal is an example of "the least bad" option haviing been selected as the best compromise- braking won't be an issue given the likely turnout speed to the fitted line.
PJW