Medium Speed Indications

New Zealand Railways use speed signalling. The medium speed indication is used when a train must travel at less than normal line speed - typically through a turnout. In the 1988 Rule Book medium speed is defined as 25 km/h, although speed boards can be used to change this speed.

This page documents the signalling at Trentham in August 2001, using down trains as an example. At Trentham medium speed is used to signal a down train from the single to the double line. In 2007 control of Trentham was transferred to Train Control and there were changes to the arrangements described here.

In some situations an installation does not follow speed signalling principles. An example is the Wellington Junction Up Main where the signals effectively indicate the route rather than the speed.




Medium Speed at Trentham in 2001 - Down Trains

Station Switched Out
Down trains cross to the double line at turnout 18. Signal 8 displays red/green (Medium Speed - 25km/h) for this move. The definition of Medium Speed includes the clause "unless a speed board authorising a higher speed is exhibited". At 18 points a speed board authorises 50 km/h. Signal 3 displays yellow/green which warns that the signal in advance is displaying Medium Speed. (Upper diagram).

Station Switched In
Trentham is switched in on race days and the Racecourse Platform is used. The cross over is made at turnout 13. Signal 3 displays Medium Speed for this move but there is no speed board. The speed allowed is therefore 25km/h. (Lower diagram).

Potential Confusion
The arrangement at Trentham is similar to that at Te Kauwhata, where there have been two serious derailments due to signals being misread. Turnouts 13 and 18 have different authorised speeds but both are signalled with a medium speed indication. At Trentham the operating pattern is different. The racecourse platform is used only when Trentham is switched in and the only trains to be signalled into the Racecoure platform are stopping EMUs.




Switched Out

3 Down Home

Signal 3 Down Home is showing yellow/green - Caution Normal Speed. Prepare to reduce to Medium speed.

In the 1988 Rule Book this means:

Proceed at Normal speed but prepared to reduce to Medium speed at next signal. Section is clear and signal in advance is at "Caution" or "Clear" for Medium speed only.

At this location this means that the route is set for the train to cross from the single line to the down main at 18 points at the south end of the Trentham station.

8 Down Starting From Main

Signal 8 Down Starting From Main is showing red/green - Clear Medium Speed.

This means:

Proceed at Medium speed. Section is clear but suitable for Medium Speed only, and signal next in advance is at "Caution" or "Clear" for medium, intermediate or normal speed.

Medium Speed is defined in the 1988 Rule Book as 25km/h "unless a speed board authorising a higher speed is exhibited".

18 points are set for the crossover to the down main.

The speed board authorises 50 km/h through the turnout. Note that a flashing lower unit would indicate Intermediate speed which is also 50km/h. Note also that the 25km/h at the north end of Trentham and the 50km/h at the south end are both medium speed in terms of the rule book.



Speed Or Route Signalling?

A down Ganz Mavag EMU crosses from the single line to the Down Main via crossover 18. Signal 8 Down Starting From Main has changed from red/green (or red/yellow) to red/red.

The speed through crossover 18 is controlled by a speed board. A green on the lower unit of signal 8 indicates that the diverging route is set. This is route signalling. Therefore Trentham uses either speed or route signalling. It all depends on which way you look at it.




Switched In (2001)

Double unit intermediate signal 3117

Displays yellow over green - Caution Normal Speed. Prepare to reduce to Medium speed. 13 points have been set for the train to enter the Racecourse Platform. 3117 is a Stop and Proceed signal.

In 2007, when control of Trentham was transferred to Train Control, 3 Down Home signal became approach controlled for movements to the loop and 3117 signal no longer displays Yellow/Green.

Signal 3 Down Home

Displays Medium Speed. The Down Starting From Loop signal (7) has not yet been cleared by the signalman so signal 3 is displaying red over yellow, rather than red over green.

In 2007, when control of Trentham was transferred to Train Control, this signal became approach controlled for movements to the loop. It was also renumbered.

7 Down Starting From Loop

has been cleared by the signalman and is indicating green.

Signal 7 is the only signal I know of in the Hutt Valley that is equipped with a marker light rather than a reflectorised disk.

In about 2007 the marker light was changed to a marker disk.




Route Signalling

The speed through crossover 18 at Trentham is controlled by a speed board. Signal 8 therefore effectively provides a route indication, so the Trentham signalling can be viewed as following either the speed or the route conventions. A location where all pretence of speed signalling has been abandoned is the Wellington Junction Up Main. The junction was rearranged in the 1960s for motorway construction. Manawatu trains, which have the normal speed indication, take the diverging route. Two notice boards change the meaning of the signals:

  • A sign specifies which route is set for each indication.
  • A speed board just before the junction restricts speed on both the straight through and diverging routes to 50km/h.



Last Updated: Sun May 19 2013

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