| Southern Pacific class AC-4 | |
| Powertype: | Steam |
| Operator: | Southern Pacific Railroad |
| Whytetype: | 4-8-8-2 |
| Operatorclass: | AC-4 |
| Driverdiameter: | 63inches |
| Cylindersize: | 24× (bore × stroke) |
| Locoweight: | 614600lb |
| Weightondrivers: | 475200lb |
| Boilerpressure: | 235psi |
| Tractiveeffort: | 112760lbf, 116900lbf rebuilt |
| Builder: | Baldwin Locomotive Works |
| Builddate: | August–October 1928 |
| Serialnumber: | 60575, 60576, 60623-60625, 60666-60669 |
| Numinclass: | 10 |
| Fleetnumbers: | 4100 - 4109 |
| Feedwaterheater: | 4-BL Worthington |
| Firstrundate: | October 1928 |
| Disposition: | All scrapped |
Southern Pacific Railroad's AC-4 (meaning Articulated Consolidation) class of steam locomotives was the first class of 4-8-8-2 cab forward locomotives. They were intended to improve on the railroad's MC (Mallet-Consolidation) class 2-8-8-2 locomotives with a larger firebox, hence, the four-wheel leading truck (instead of the two-wheel).
The AC-4s were the first SP Mallets built for simple expansion. Baldwin Locomotive Works built them in August through October 1928 with a maximum cutoff of 70%, so tractive effort was rated at 112760lbf; a few years later, limited cutoff was dropped and calculated tractive effort increased to 116000lbf.
The AC-4s were removed from service starting in 1953, and all ten were scrapped by June 1955.