| Southern Pacific class MC-6 | |
| Powertype: | Steam |
| Builder: | Baldwin Locomotive Works |
| Serialnumber: | 38523–38534, 38711–38713, 39673, 39731, 39857, 39858, 39874 |
| Builddate: | October–December 1912 and April–May 1913 |
| Totalproduction: | 20 |
| Whytetype: | 2-8-8-2 Mallet |
| Uicclass: | (1′D)D1′ h4v |
| Driverdiameter: | 570NaN0 |
| Axleload: | 512001NaN1 |
| Weightondrivers: | 400700lb |
| Locoweight: | 435800lb |
| Locotenderweight: | 615200lb |
| Fueltype: | Fuel oil |
| Fuelcap: | 3200usgal |
| Watercap: | 10000usgal |
| Boilerpressure: | 2002NaN2 |
| Firearea: | 60.4square feet |
| Totalsurface: | 4177square feet |
| Superheaterarea: | 5016square feet |
| Cylindercount: | Four |
| Hpcylindersize: | 26x |
| Lpcylindersize: | 40x |
| Valvegear: | Walschaerts |
| Tractiveeffort: | 948802NaN2 |
| Operator: | Southern Pacific Company |
| Operatorclass: | MC-6 |
| Fleetnumbers: | 4029 - 4048 |
| Firstrundate: | December 17, 1912 |
| Retiredate: | 1946 - 1949 |
| Disposition: | All rebuilt as AC-3 class; all of which were scrapped |
Southern Pacific Company's MC-6 class of steam locomotives is made up of two batches: the first consisting of 15 locomotives weighing 4358001NaN1 built in 1912, the second consisting of 5 locomotives weighing 4371001NaN1 built in 1913. All of the locomotives were built by Baldwin Locomotive Works. This was the last class of 2-8-8-2 locomotives that Southern Pacific (SP) ordered as cab forward locomotives.
Locomotive number 4043, the newest of the early class of MC-6 locomotives, was displayed at the Panama–Pacific International Exposition along with SP's first locomotive, C. P. Huntington.
All but three of the locomotives in this class were rebuilt as the AC-3 class with "simpled" uniform cylinders of 22x by 1930. The last three were thus rebuilt in 1937. During the rebuild, 4¼-BL Worthington feedwater heaters were also installed on the fireman's side of the locomotives.
After their rebuilds, the locomotives were used through the end of World War II with the last one scrapped on August 20, 1949.