Southern Pacific class AC-2

{{one source|date=July 2016}}

{{Infobox locomotive

| name=Southern Pacific class AC-2

| powertype=Steam

| operator=Southern Pacific Railroad

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

| operatorclass=AC-2

| fleetnumbers=4017 - 4028

| numinclass=12

| driverdiameter ={{convert|57|in|0|abbr=on}}

| cylindersize={{convert|22|x|30|in|0|abbr=on}} dia × stroke

| locoweight={{convert|481,200|lb|abbr=on}}

| weightondrivers= {{convert|440800|lb|abbr=on}}

| boilerpressure= {{convert|210|psi|abbr=on}}

| tractiveeffort= {{convert|90940|lbf|kN|abbr=on}}

| feedwaterheater=4{{frac|1|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 {{convert|5000|lbf|kN}} in tractive effort, but also increased the locomotives' weight by about {{convert|50,000|lb}}. The locomotives were deemed successful and remained in active service until after World War II.