In 2005 GWRC unveiled a plan for the upgrade of the Wellington commuter rail system to increase capacity and service frequencies.

[6] GWRC formed a subsidiary named Greater Wellington Rail Limited to purchase the EMUs and three tenders were shortlisted; Construcciones y Auxiliar de Ferrocarriles (CAF), a consortium of Rotem & Mitsui and a consortium of EDi Rail & Bombardier transportation.

[17] The interior configuration allows for more standing room, increasing the passenger capacity compared with the EM/ET class units.

The preliminary design was modified with the addition of an emergency exit door at each end to allow for evacuation in the single-track tunnels on the Wellington network (seven on the Johnsonville Branch and five on the North–South Junction).

On the EM/ET units, drivers had to double-check the "doors closed" light was on before applying power, potentially leading to human error.

This configuration led to an incident in March 2013 when three EM/ET units left Wingate station with their doors open and no passenger staff on board, after the driver mistakenly thought he heard the "right-of-way" buzzer.

[20] A large amount of preparation works were undertaken in the Wellington region to allow the units to operate: clearances in tunnels, at platforms and under some bridges had to be increased to take the new trains.

[11] KiwiRail installed eleven new rectifier substations to increase the electrical supply for the new trains (nine on existing electrified lines, and two on the new extension to Waikanae), and hardened the signalling system against interference from their AC traction equipment.

[31] The lower-floor trailer unit has an area with folding seats for wheelchairs or prams, and for three bicycles during off-peak (and during peak if in the non-peak direction of travel).

[33] Although squealing caused by wheel slip is not uncommon on rail systems worldwide, the gradient and curve radius on the Johnsonville Line exacerbated it.

During the 2012-13 Christmas/New Year period, dispensers of friction modification fluid (oil) were fitted to the line to lubricate the rail and help reduce the noise.

The fluid has produced mixed results, reducing the squeal in some places but making no difference on others, however has the unfortunate side-effect of reducing rail adhesion – when combined with a wet rail, it has caused trains to overshoot platforms and to lose traction, resulting in cancellations or delays.

The class usually wears an unpainted stainless steel finish, offset by the Metlink branding colours of dark blue and lime green, with yellow safety ends.

It was nominated by Linden commuter Brian Bond, chosen for Wellington's windy reputation and the new trains being "as fast as the wind" and a "breath of fresh air to the transport system".

The name was also chosen as it is easy to pronounce and spell, is distinctively Kiwi, and had support from local iwi (Māori tribes).