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California's Railroad to the U.S. 1861 - 1996

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Model Train Help EBook

Southern Pacific
Articulated
Steam Locomotives



Class# SeriesBuilderTotal
SP MC-12-8-8-24000-4001Baldwin(2)
SP MC-22-8-8-24002-4016Baldwin(15)
SP MC-42-8-8-24017-4028Baldwin(12)
SP MC-62-8-8-24029-4048Baldwin(20)
TNOMM-12-6-6-2950-961 Baldwin(12)
SP MM-12-6-6-2900-911 Baldwin(12)
SP MM-24-6-6-24200-4211Baldwin(12)
SP MM-32-6-6-23930-3931Schenectady(2)
SP AM-22-6-6-24200-4211rebuilt from MM-2
SP AM-24-6-6-23900-3911rebuilt from MM-2
SP AC-12-8-8-24000-4016rebuilt from MC-1 and MC-2
SP AC-22-8-8-24017-4028rebuilt from MC-4
SP AC-32-8-8-24029-4048rebuilt from MC-6
SP AC-44-8-8-24100-4109Baldwin(10)
SP AC-54-8-8-24110-4125Baldwin(16)
SP AC-64-8-8-24126-4150Baldwin(25)
SP AC-74-8-8-24151-4176Baldwin(26)
SP AC-84-8-8-24177-4204Baldwin(28)
SP AC-92-8-8-43800-3811Lima(12)
SP AC-104-8-8-24205-4244Baldwin(40)
SP AC-114-8-8-24245-4274Baldwin(30)
SP AC-124-8-8-24275-4294Baldwin(20)

282 units built between 1909 and 1944.

MC = Mallet Consolidation

MM = Mallet Mogul

AM = Articulated Mogul

AC = Articulated Consolidation

Mallet comes from the designer of articulated compound locomotives one Anatole Mallet. To be a true Mallet locomotive, it must be articulated and be a compound, i.e. the steam is used twice in separate cylinders. Mallet is pronounced Malley...

A simple explanation of compounding follows:
The high pressure cylinders use the steam first and are the smaller of the two sets, the steam is then passed to the larger low pressure cylinders where the steam is used again. It was supposed to be more efficient, but the complication was maintenance intensive, and the locomotives were actually slower than simple expansion locomotives. Which is why SP converted the compounds to simple expansion in later years.

SP lingo was that all articulateds were known as Mallets regardless of whether they were simple or compound.

The AC's were built as simple expansion locomotives, so the A designation in the class descriptor.

On the SP, a 2-8-0 was known as Consolidation, hence the 'C' in the x-8-8-x drivered locos and the 2-6-0 were known as Mogul's, hence the M in the x-6-6-x locomotives.


References:

  • A Century of SP Steam Locomotives - Guy L. Dunscomb
  • Cab-In-Front (The Half Century Story of An Unconventional Locomotive) by John.B.Hungerford © 1959
    • reprinted by PFM 1974
    • SP Co. Steam Locomotive Compendium by Diebert & Strapac
    • Southern Pacific Steam Pictorial Vol.2 by Dunscomb, Dunscomb & Pecotich
    • Volume 7 -- Southern Pacific Articulated 4-8-8-2 AC-4 to AC-6 Pictorial - by Duane Karam Jr. & Jeff Ainsworth - Monte Vista Publishing
    • Volume 10 -- Southern Pacific Articulated 4-8-8-2 AC-7 and AC-8 Pictorial - by Duane Karam Jr. & Jeff Ainsworth - Monte Vista Publishing
    • Volume 13 -- Southern Pacific Articulated 4-8-8-2 AC-10 to AC-12 Pictorial - by Duane Karam Jr. & Jeff Ainsworth - Monte Vista Publishing
    • Volume 15 -- Southern Pacific Articulated AC's and MC's Pictorial - by Duane Karam Jr. & Jeff Ainsworth - Monte Vista Publishing
    • Volume 19 -- Southern Pacific Misc. Mallets Pictorial - by Duane Karam Jr. & Jeff Ainsworth - Monte Vista Publishing

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Saturday, March 08, 2003