 Their wheel arrangement is described by the Whyte notation 2-6-4T and the first members of the class entered service in 1904.The locomotives were tank engines designed by the Railways Department's Chief Mechanical Engineer A. L. Beattie, and were mainly built for suburban duties such as those between Christchurch and Lyttelton.They also saw main-line service in the Taranaki region, but most of the class members were assigned to branch line and local services throughout the country.