LSWR A12 class

{{Use dmy dates|date=June 2017}}

{{Use British English|date=June 2017}}

{{Infobox Locomotive

|name=LSWR A12 class

|powertype=Steam

|image= File:Nine Elms Locomotive Works, 256, London and South Western Railway (L&SWR) 539, Jubilee-class (Ans 05373-0488).jpg

|caption=LSWR A12 class No. 539 in 1888

|gauge={{RailGauge|ussg}}

|whytetype=0-4-2

|uicclass=B1 n2

|designer=William Adams

|builder=LSWR Nine Elms Works (50)
Neilson & Co. (40)

|builddate=1887–1894

|totalproduction=90

|driverdiameter={{convert|6|ft|1|in|m|3|abbr=on}}

|trailingdiameter={{convert|4|ft|0|in|m|3|abbr=on}}

|cylindercount=Two, inside

|cylindersize={{convert|18|x|26|in|mm|0|abbr=on}}

|boilerpressure={{convert|160|lbf/in2|MPa|2|abbr=on|lk=on}}

|tractiveeffort={{convert|15690|lbf|kN|2|abbr=on|lk=on}}

|operator=London and South Western Railway
Southern Railway
British Railways

|retiredate=1928–1948

|operatorclass=LSWR: A12
SR: A12
BR: 1MT

|nicknames=Jubilees

|disposition=All scrapped

}}

The A12 locomotives of the London and South Western Railway were built between the years 1887 and 1895 to the design of William Adams. Ninety of the locomotives were built, fifty at Nine Elms Works and forty by Neilson and Company, although the latter together with the final twenty from Nine Elms were officially known as the O4 class. They were unusual for their time, with a wheel arrangement of 0-4-2. This arrangement was used by few of the other railway companies and never proved popular (although the Great Northern Railway had 150 such locomotives). They bore the nickname "Jubilees", because the first batch appeared in the Golden Jubilee of Queen Victoria's reign.