| Southern Pacific class AC-2 | |
| Powertype: | Steam |
| Operator: | Southern Pacific Railroad |
| Operatorclass: | AC-2 |
| Fleetnumbers: | 4017 - 4028 |
| Numinclass: | 12 |
| Driverdiameter: | 57inches |
| Cylindersize: | 22x dia × stroke |
| Locoweight: | 481200lb |
| Weightondrivers: | 440800lb |
| Boilerpressure: | 210psi |
| Tractiveeffort: | 90940lbf |
| Feedwaterheater: | 4-BL Worthington |
| Retiredate: | 1947 - 1948 |
| Disposition: | scrapped |
| Whytetype: | 2-8-8-2 |
Southern Pacific Railroad's AC-2 class of steam locomotives was the second in the AC series of cab forward locomotives built for Southern Pacific (SP). This class consisted of locomotives that were rebuilt from Baldwin-built SP MC-4 class locomotives by 1930.
The rebuild from MC-4 class gained the locomotives about 5000lbf in tractive effort, but also increased the locomotives' weight by about . The locomotives were deemed successful and remained in active service until after World War II.