Signalling

Like most signalling systems in the world, New Zealand signalling is based on speed signalling, where the aspect of a signal tells the driver what speed to proceed at. Other different types of rules, such as Centralised Train Control (CTC) and Automatic Signalling Rules (ASR) define how signals work and how they are interpreted. This page mostly deals with ASR as used in the Auckland suburban network between Papakura and Swanson, but most information can also be applied to other signalling rules (albeit with some slightly different terminology).

Please note that none of the information below applies to Track Warrant Control (TWC) areas, as this system uses points indicators rather than signals. Points indicators do not convey authority to proceed and as such are not covered here.

Signal aspects

The simplest signals can display three aspects: green (top), yellow (middle) and red (bottom). Some signals can also display a fourth aspect, flashing yellow (also in the middle). Signals with two heads can display a wider variety of aspects, each of which are explained below. Aspects are typically described by the colour of the top head followed by the colour of the bottom head, e.g. Green over Red for Clear, Normal Speed.

Note: signal aspects not displayed in this table (such as Green over Green) are not valid. Signals displaying an invalid aspect are to be treated as Stop.

Aspect	Meaning	Common usage
	Clear The track is clear ahead, train may proceed at normal speed	Used when the line is clear ahead and the train may travel at the line's, or rolling stock's, speed limit
	Advance Caution The next signal is at Caution: prepare to stop at signal after next	Advance warning of a signal displaying Stop; the next signal is displaying Caution
	Caution The next signal is at Stop	Warning of a signal displaying Stop. Can also indicate the next section of track is clear until the end of the line or an All Trains Stop board.
	Stop The train must not proceed	Used when another train is in the section ahead, or the points are not set for the train. May be passed in some circumstances; see Signal types
	Clear, Normal Speed The track is clear ahead, train may proceed at normal speed	Used when the line is clear ahead and the train may travel at the line's, or rolling stock's, speed limit
	Advance Caution, Normal Speed The next signal is at Caution, Normal Speed: prepare to stop at signal after next	Gives the driver plenty of warning of an upcoming signal at Stop; the next signal is displaying Caution, Normal Speed
	Caution, Normal Speed The next signal is at Stop	Warning of a signal displaying Stop. Can also indicate the next section of track is clear until the end of the line or an All Trains Stop board.
	Stop The train must not proceed	Used when another train is in the section ahead, or the points are not set for the train. May be passed in some circumstances; see Signal types
	Clear, Medium Speed The track is clear ahead but is suitable for medium speed only	Often used when the train is switching tracks or taking a diverging route
	Caution, Medium Speed The next signal is at Stop, and track ahead is suitable for medium speed only	Not often seen, but can be seen when the train switches tracks or takes a diverging route then must stop at the next signal
	Advance Caution, Normal Speed Proceed at normal speed, prepare to reduce to medium speed at signal after next	Seen when the train will switch tracks or diverge at the signal after the next one (which is set to medium speed), and the track until then is suitable for normal speed.
	Caution, Normal Speed Proceed at normal speed, prepare to reduce to medium speed at next signal	Seen when the train will switch tracks or diverge at the next signal (which is set to medium speed), and the track until then is suitable for normal speed.

Speeds

Normal Speed

Indicates that the line ahead is clear and suitable for line speed, i.e. the maximum authorised speed on the section of track.

Medium Speed

Indicates that the line ahead is suitable for medium speed only, but points are set correctly and the line is clear. By default, this is a maximum speed of 25km/h, unless a speed board or speed indicator authorising a higher speed is displayed next to the signal.

Low Speed

Indicates that the points are set correctly, but the track ahead may be occupied. The locomotive engineer must be able to stop short of any obstruction. Unlike Medium Speed, Low Speed is fixed at a maximum speed of 25km/h for all signals.

Subsidiary lights

Signals may have any one of these subsidiary lights.

Name	Description	Uses
"A" light	When lit, converts a Stop and Stay signal into a Stop and Proceed signal	"A" lights are lit when a signal box is switched out - it indicates the points are correctly set for either the mainline, loop or branch, but does not guarantee the track is clear. It is also lit if the axle counters fail and have lost track of trains (the next train through resets the signals, a "sweep"), and can also be used to facilitate train rescues in the event of a breakdown or a train is otherwise disabled.
Low speed light	When lit, indicates Low Speed (maximum 25km/h and track may be occupied)	Within station limits, low speed lights have similar uses as the "A" light, i.e. they can be used for train rescues or to reset axle counters after failure. The points are in the correct position when this light is lit, but the track may be occupied. At Britomart Transport Centre and Wellington Station, low speed lights are used to signal trains onto platforms that are already occupied, perhaps because two trains need to couple or all platforms are already occupied, and the incoming train must get in. On the Johnsonville Branch, low speed lights are used to signal trains onto an opposing main (for example, a Down train onto the Up Main at Khandallah) whether or not the track is occupied. Such use is specific to this line.
Restricted speed light	When lit trains may pass at a maximum speed of 40km/h	Same uses as the low speed light, but is found on departure signals instead
Loop light	Points are set for loop or diverging route	Used on signals at the start of Track Warrant Control zones - when lit, the points are set for the loop at stations, or the diverging route at junctions
Electrified light	When lit, the set route is electrified	Found on signals which can direct to both electrified and non-electrified tracks; examples are at Papakura and Waikanae. If a signal has this light fitted but not lit, electric trains must not pass the signal even if it otherwise allows trains through (i.e. not at Stop). May be found with any other subsidary lights.
Marker light	Permanent red aspect	Used instead of a marker on single head signals (see below) in dark areas, such as Britomart Tunnel. Can also be used above a signal head to indicate a permanent medium speed aspect - ordinary marker plates are not used above signal heads.

Signal types

Signals generally come in three different classifications: Stop and Stay, Stop and Proceed, and Departure. The classifications relate to the procedures used when passing a signal at Stop. Note that, unlike a car, a train may pass a red signal as long as the driver is following the appropriate rules.

Signals can also come in single-head and double-head varieties. The following table explains the difference between signals.

Classification	Identifying features	Passing at Stop	Typical uses
Absolute Stop and Stay	Red marker or second head directly underneath top head	Verbal authorisation by Train Control	Almost anywhere - used as directing signals (with a Low Speed light) as well as intermediate signals (with an "A" light). Double head signals are almost always found controlling turnouts, and indicate the permitted speed for the set route.
Permissive Stop and Proceed	Red marker or second head below and to the right of top head	Train must stop for 10 seconds, then the driver may proceed at such a speed that they can stop in half the visible distance	Single head permissive signals are used on double line block sections (outside bi-directional areas) as intermediate signals, allowing greater capacity than with absolute signals alone. In Auckland, double head permissive signals are used on the opposing main (e.g. for a Down train running on the Up Main) as an advance warning of the upcoming absolute signal at the end of the block section.
Departure	Like absolute, with blue Depart sign on signal number plate	A SWA1 (Auckland, ASR) or Mis. 59 (elsewhere) form must be issued	Authorises entry into a block section from interlocked stations

Route and speed indicators

Some signals may have a route indicator, speed indicator or both. Route indicators are typically found below the signal heads and tell the driver which route the train is taking, shown with one or two characters. The meaning of the characters is specific to each signal, but usually follows conventions such as "D" for Down Main.

PAP 2220 signal with route indicator below signal heads — Signal PAP 2220, with the route indicator at the bottom showing "U" for Up Main. This particular route indicator only shows this and "D" for Down Main, but others can display many more indications than this, usually with two letters such as "D1" (Down Main on the NAL) on signal WSF 1504

Speed indicators are usually above the signal heads, and indicate to the driver the speed they may travel at when a signal is set to any aspect other than Clear or Stop.

Unusual signals

Of course where there are rules, there are exceptions! Some signals aren't quite the same as others.

Photo	Signal identification	Features
	PAP 2231	Papakura Up Main (northbound track) signal for southbound trains - signal is permanently at Stop as this marks the end of ASR and bi-directional signalling
	MNK 1904	Manukau Platform 2 departure - upper signal head has no green light as Clear, Normal Speed is not possible, as two signals ahead is the junction with the North Island Main Trunk, suitable for medium speed only. The lack of a lens is visible in the large version of the photo to the left (click to enlarge).
	AKD 127, AKD 129, AKD 34, AKD 36	Auckland inbound from Orakei (opposite port sidings) Up and Down Mains, Auckland outbound (near Vector Arena footbridge) Up and Down Mains - route indicators above the signals heads due to clearance issues

Changelog

23 May 2019: Clarified the differences between Medium and Low Speeds
20 October 2014: Add brief reference to Track Warrant Control, fixed image
11 February 2014: Removed references to intermediate speed aspects
23 January 2014: Improved mobile version, fix a light image
20 January 2014: Launch signalling page

PLEASE NOTE

This information is not official, and should not be taken as completely correct or complete. Corrections to this information are welcome through email.