High speed routes where braking distances require further cautioning to be provided are equipped with flashing double yellow and flashing single yellow aspects (dependant upon location and need) which are intermediate stages and mean, respectively, expect the next signal to be showing steady double yellow or steady single yellow. When an additional stage of caution is required on higher speed or heavily-trafficked lines a double yellow means "preliminary caution" and in effect means expect the next signal to show single yellow. Red, yellow and green mean respectively stop, proceed at caution and clear to proceed at maximum permitted speed The comparison with UK signalling has been accurately stated other than the reference to "triple yellow" there is no such thing. G - There is a slight flaw in the design of the low speed light lights in the Sydney, Strathfield and Sydenham areas the low speed light goes out before a heart-stopping moment later, the train stop operates and the Top Green light lights up what should have been done, is to keep the low speed light on, until the train stop fully operates, when the Top Green comes on at the same time as the low speed light goes out. * with white markers, the marker light is always lit (from 1957 to last one replaced at Gosford in about 2000).į - double light signals at the transition between double and single aspects MAY be lamp proved, but am unsure of details,Į - except for timed low speed signals at say Central, the top green light will never (not often) clear if the train stop fails to clear, and the previous signal is limited to caution (G/R) * with red markers, the marker light is lit if the main head fails (after about 1968) * Am not sure about latest practice with double light signals.Ĭ - as far as I know this has never been used, as it would be to restrictive.ĭ - in single light territory, the previous signal is restricted to "Yellow" if the lamp at the next signal fails. * In Single light areas the lamps are 25W with a filament change over relay in the lamp case lighting a spare 25W filament, with detection * In Double light areas the original lamps used to have 2W + 2W fillaments in parallel, with no detection ![]() "Northmetro"Ī - never used as axuliary filament is always of same power in both double and single light territory, i.e. ![]() Not only is this the real protection for a train if a lamp fails, but often the signal aspects are actually controlled by the trainstop position to avoid a train being tripped at a failed trainstop. Hopefully driver reports fault when light goes dimī - auxiliary filament - same power, lit when main filament fails, alarmed to control pointĬ - confirming next signal is lit before clearing previous signalĭ - restricting aspect of previous signal when lamp failsĮ - Also remember that many urban systems still have trainstops. Not only is this the real protection for a train if a lamp fails, but often the signal aspects are actually controlled by the trainstop position to avoid a train being tripped at a failed trainstop.Ī - auxiliary filament (lower power under-run - lit at same time of main filament). restricting aspect of previous signal when lamp failsĪlso remember that many urban systems still have trainstops. confirming next signal is lit before clearing previous signal auxiliary filament - same power, lit when main filament fails, alarmed to control point Hopefully driver reports fault when light goes dim auxiliary filament (lower power under-run - lit at same time of main filament). Obviously with a single light, the consequences of lamp failure could be higher (although not so much of a problem today with LED signals) so most designs have one or more of the following additional features When UK went to colour light signals (which Queensland subsequently copied - probably engaged UK consultants) they simplified to a single light. ![]() The two light systems (VIC, NSW metro etc.) were originally designed to replicate the earlier semaphore signals, where the position of the arm was the primary indication but a light was necessary for night time. ![]() All the above contributions demonstrate that it's very easy to get a simple system to become more complex as you try to cover more situations, but attempt to accommodate it within the existing signal profile.
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