Auckland’s population is expected to grow rapidly, with the surrounding infrastructure requiring improvement to support the increasing demand. There are more than 21 million commuter rail journeys on the network in Auckland each year. The network also carries freight traffic to support the national freight supply chain.


With the growing demand on the New Zealand rail network, KiwiRail needed to increase the efficiency of their rail systems. Currently the network is a 25kV AC Rail Return (RR) system which is efficient for the current network size, however, when expanded the RR system becomes inefficient. KiwiRail are exploring a 25kV AC Auto Transformer topology that will maintain a high efficiency on a large network but will require assistance for planning the staged implementation of the new system.


Middleton Group undertook a high-level desktop review of the present traction power system on the Auckland network and proposed a simple low maintenance 25-0-25 kV AC Auto Transformer system retrofit. The proposed retrofit included a plan for an AT system stretching from Auckland to Palmerston North. The study identified key locations for infrastructure and provided staging options for the adoption of the AT system while migrating from the existing RR system to not disrupt existing transport options.


The detailed study performed by Middleton Group on the viability and planning of an AT system for the New Zealand rail network gave KiwiRail the confidence to pursue this option for the entire north island rail network.

The implemented AT system will result in higher efficiencies across the network and will provide all passengers and freight with an easier mode of transport across the north island of New Zealand.