| NZR UD class | |
| Powertype: | Steam |
| Builder: | Baldwin Locomotive Works |
| Builddate: | 1904 |
| Totalproduction: | 2 |
| Whytetype: | 4-6-0 |
| Driverdiameter: | 58inches |
| Weightondrivers: | 29.2LT |
| Locoweight: | 39.5LT |
| Tenderweight: | 27.4LT |
| Fueltype: | Coal |
| Watercap: | 2400impgal |
| Tendercap: | 5LT |
| Cylindercount: | 2 |
| Cylindersize: | 16.5x |
| Valvegear: | Stephenson |
| Firearea: | 16.5square feet |
| Boilerpressure: | 1850NaN0 |
| Totalsurface: | 1095square feet |
| Tractiveeffort: | 15280lbf |
| Retiredate: | 1929 |
| Numinclass: | 2 |
The NZR U class was a class of two 4-6-0 steam locomotives built by the Baldwin Locomotive Works in 1904 for the Wellington and Manawatu Railway Company. When that company was nationalised in 1908, they passed into the ownership of the New Zealand Railways and received the designation U.
The two locomotives were the final new motive power ordered by the independent Wellington and Manawatu Railway Company.
Weighing 39.5LT with a tender of 27.4LT, they could haul express trains easily at 50mph on track that was flat or only a light grade. The 58inches coupled driving wheels were large for the period, and were the largest to run in New Zealand. Their working steam pressure was 185psi, and they had 16.5inches diameter cylinders with 22inches piston strokes.
The handsome UD locomotives were more than capable of making up lost time, with mile-a-minute runs recalled with pride by Manawatu railway employees. Due to their power, the locomotives were capable of hauling mail trains without the need for a banking locomotive from Paekakariki south to Pukerua Bay, and were used principally on mail trains north of Paekakariki.
Although they were fast and powerful, the UD locomotives were unable to survive a programme of standardisation undertaken between 1925 and 1935 to eliminate small locomotive classes that were costly to maintain in favour of large, homogeneous types that provided economies of scale. They were written off in 1929.