Southern Pacific class AC-11

{{Infobox locomotive

| name=Southern Pacific class AC-11

| powertype=Steam

| operator=Southern Pacific Railroad

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

| operatorclass=AC-11

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

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

| locoweight={{convert|657900|lb|kg t|abbr=on}}

| weightondrivers={{convert|531700|lb|kg t|abbr=on}}

| boilerpressure={{convert|250|psi|MPa|abbr=on}}

| tractiveeffort={{convert|124300|lbf|kN|abbr=on}}

| feedwaterheater=6SA Worthington

| fleetnumbers=4245–4274

| builder=Baldwin Locomotive Works

| serialnumber=64677–64706

| builddate=November 1942 – April 1943

| firstrundate=November 24, 1942

| retiredate=1954–1958

| disposition=all scrapped

| whytetype=4-8-8-2

| image=SOUTHERN PACIFIC CAB FORWARD CLASS AC-11 4-8-8-2 ARTICULATED OIL BURNING LOCO DESIGN ENGINE NUMBER 4247 SOUTHBOUND BETWEEN SAGE HEN AND MADELINE, CALIFORNIA NEAR THE MODOC NATIONAL FOREST IN 1956.jpg

| caption=

}}

Southern Pacific Railroad's AC-11 class of cab forward steam locomotives was the seventh class of 4-8-8-2 locomotives ordered by Southern Pacific (SP) from Baldwin Locomotive Works;{{Harvp|Ransome-Wallis|2013|p=323}} SP was so pleased with the AC-10 class built a year earlier that the railroad began placing orders for AC-11s while the AC-10s were still being built and delivered. They were built between November 1942 and April 1943, closely resembling the AC-10s.

The first AC-11, number 4245, entered service on November 24, 1942, and the last, 4274, on May 9, 1943. SP used these locomotives for between 12 and 15 years, with the last retirements from this class occurring on September 24, 1958. Shortly after their retirement, the AC-11s were scrapped, with the last, number 4274, in April of 1959, after pulling a farewell excursion over the cab forward's former stomping ground, Donner Pass.{{Harvp|Scribbins|1997|p=93}}{{Harvp|Boyd|1992|p=3}}{{Harvp|Yenne|1994|p=98}}