a while ago I noticed that some signals have these old looking lamps instead of the LEDs. all the ones I've seen are "backward", as in the lamps are facing that track's direction of travel, so incoming trains would only see the back of the signal (useless). Maybe TfNSW prioritised upgrading only the signals that are actually used, and left these ones in because they're not necessary.
photo taken at Hornsby on the south end of a platform (forgot which). nice double crossover there in the background.
that's a great idea, thanks