Southern Pacific class MC-4

{{one source|date=July 2016}}

{{Infobox locomotive

| name=Southern Pacific class MC-4

| powertype=Steam

| image=Southern Pacific cab forward mallet locomotive 4030.jpg

| operator=Southern Pacific Railroad

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

| operatorclass=MC-4

| whytetype=2-8-8-2

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

| hpcylindersize={{convert|26|x|30|in|0|abbr=on}}

| lpcylindersize={{convert|40|x|30|in|0|abbr=on}}

| locoweight={{convert|432600|lb|abbr=on}}

| weightondrivers={{convert|398500|lb|abbr=on}}

| boilerpressure={{convert|200|psi|MPa|abbr=on}}

| tractiveeffort={{convert|85040|lbf|kN|2|abbr=on}}

| fleetnumbers=4017 – 4028

| builder=Baldwin Locomotive Works

| serialnumber=36490-36493, 36524-36527, 36614-36616, 36634

| builddate=April–May 1911

| firstrundate=July 21, 1911

| retiredate=1947 – 1948

| disposition=scrapped

}}

Southern Pacific Railroad's MC-4 class of steam locomotive was the second class ordered and built as cab forward locomotives. Southern Pacific (SP) found the MC-2 class sufficient for a proof-of-concept for cab forward locomotives and sought to continue with now tested designs.

In service, SP found that the locomotives were a little too slow for the traffic needs. By 1930, all of the MC-4 class had been "simpled" to use uniform size cylinders and feedwater heaters were installed to address the issue. They were then reclassified as AC-2 locomotives. As rebuilt, the locomotives continued in service through the end of World War II.