I have always wondered why Signals are sometimes placed before a platform rather than having them AFTER (non Interlocked locations).
Generally headway is maximised if there is
*A departure signal just past a platform; 20m allows the stationary driver to see this signal without need for a repeater. 20m is about one carriage length.
*B arrival signal a full overlap before that platform. At 80km/h this overlap would be about 400m.
*C additional signals between departure and arrival signals if the section is "long".
*A1 At Hornsby, the up departure signals from Platforms 1 & 2 are right next to the platform, and repeaters are required. This distance cannot be increased because the scissors crossovers are in the way.
*A2 At Hornsby, the down departure signals from Platforms 1 & 2 were actually inside the platforms, because of crossovers, with the recent changes have moved these crossovers out of the way and these signals and crossovers are about 20m away, eliminating the repeaters.
*At Pymble, the Down departure signals are perhaps 100m from the platform, not sure why.
* Between Blacktown and St Marys Up and Down signals are at the same location. At Mt Druitt, one pair of signals are at the preferred departure end of the platform, while the other pair is at the arrival end of the platform. Perhaps this save design costs, but it is stupid nevertheless.