It was the only Mallet locomotive to operate for the NZR The Rimutaka Incline opened in 1878, connecting Wellington with the Wairarapa region, and with the completion of the Wairarapa Line in December 1897, it provided NZR's main link to the north as the west coast route was then privately owned by the Wellington and Manawatu Railway Company (WMR).

Six special Fell locomotives, the H class, worked the Incline, but after 1897, traffic increases necessitated additional motive power.

[1][3] To meet Beattie's requirements, Pearson designed E 66 as a Mallet articulated locomotive with a wheel arrangement of 2-6-6-0T under the Whyte notation system.

[3] This compounding encouraged the use of a then remarkably high boiler pressure, 200 psi, which in turn caused the use of a corrugated furnace for strength.

[4] The locomotive was built at the Petone Railway Workshops in the Hutt Valley under Pearson's direct supervision and entered service on 23 February 1906.

E 66 consumed 167 pounds of coal per mile, while an H class used 117 pounds, and its operating cost was 54.10 pence per mile compared to 37.73 pence for an H. Furthermore, E 66's crews complained of excessive heat in the cab, especially through tunnels,[2] and firemen had to wear heat-resistant asbestos pants due to the excessive heat.

[1] In December 1908, the WMR was purchased by NZR and its Wellington - Manawatu Line was incorporated into the national network as part of the North Island Main Trunk Railway (NIMT).