Southern Pacific class AC-4

{{one source|date=July 2016}}

{{Infobox locomotive

| name=Southern Pacific class AC-4

|image=Southern Pacific forward cab mallet locomotive 4100.jpg

|caption=Southern Pacific 4100 and its crew

| powertype=Steam

| gauge={{track gauge|ussg|allk=on}}

| operator=Southern Pacific Railroad

| whytetype=4-8-8-2

| operatorclass=AC-4

| driverdiameter={{convert|63|in|0|abbr=on}}

| cylindersize={{convert|24|×|32|in|0|abbr=on}} (bore × stroke)

| locoweight= {{convert|614600|lb|kg t|abbr=on}}

| weightondrivers={{convert|475200|lb|kg t|abbr=on}}

| boilerpressure={{convert|235|psi|MPa|abbr=on}}

| tractiveeffort={{convert|112760|lbf|kN|abbr=on}}, {{convert|116900|lbf|kN|abbr=on}} rebuilt

| builder=Baldwin Locomotive Works

| builddate=August–October 1928

| serialnumber=60575, 60576, 60623-60625, 60666-60669

| numinclass=10

| fleetnumbers=4100 – 4109

| feedwaterheater=4{{frac|1|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 {{convert|112760|lbf|kN|abbr=on}}; a few years later, limited cutoff was dropped and calculated tractive effort increased to {{convert|116,000|lbf|kN|abbr=on}}.

The AC-4s were removed from service starting in 1953, and all ten were scrapped by June 1955.