The class Z of the Northern Pacific were 2-6-6-2 Mallet locomotives built for low-speed helper service which were virtually identical to the Great Northern's class L-1. They were among the first of this wheel arrangement. Compared to the 0-6-6-0, they still had a high adhesive weight, but better guidance in tight curves. They had a Belpaire firebox, 55 inch drivers and cylinders with diameters of 21.5 and 33 inches at a stroke of 32 inches.

In 1907, 16 were built by Baldwin. They were rated at 850 tons on a 2.2 percent incline. In service it turned out that they showed leakages in various places. In 1913, five were converted to oil burning and in 1922 and 1923, three more had followed. But in 1918, one of these had already been converted back to coal firing and by 1925, all were again coal fired. Twelve were retired in 1926 and 1927 and the last one survived until May 1939.