Over the course of 1901–1903, five more Bs were built in Addington Workshops, with the last entering service in May 1903. The locomotives were advanced for their time, featuring a new piston valve design and a modified form of Walschaerts valve gear, and they were designed to haul freight trains on flat lines and on the hilly section of the Main South Line between Oamaru and Dunedin. For the time, these were quite significant figures. The Addington engines were unusual in the fact that they employed a screw reverse configuration, instead of the standard reversing lever. They also had fold-down seats for both driver and fireman.