Southern Pacific
Articulated
Steam Locomotives
The SP had 282 articulated units built between 1909 and 1944. Not all operated on the SP at the same time.
MC = Mallet Consolidation
MM = Mallet Mogul
AM = Articulated Mogul
AC = Articulated Consolidation
Mallet comes from the name of the designer of articulated compound locomotives, Anatole Mallet (pronounced "Malley" and not mallet). To be a true Mallet locomotive, it must be both articulated and a compound steam loco, i.e. the steam is used twice in separate cylinders.
Compound steam use, simply described, is double the use of a release of steam into the cylinders, with it first going to the high pressure cylinders, which are the smaller of the two sets of cylinders. When expelled from the high pressure cylinders, the steam is then passed to the larger low pressure cylinders, where the steam is used again but having lost some of its force. Compound steam, in theory, was supposed to be more efficient. In reality, compound operations were more maintenance intensive. This added maintenance, along with compound steam locomotives being slower than simple expansion locomotives, is why the SP, and most other railroads, converted their compound locomotives to simple expansion in later years.
SP lingo was that all articulateds were known as Mallets regardless of whether they were simple or compound.
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.
The AC's were built as articulated locomotives with simple expansion, so the A designation in the class descriptor.
The cab forward steam locomotive design was used only by the SP. No other railroad adopted this manner of a cab at the front end of a locomotive, although the Western Pacific did consider it. The cab forward design came about to address the issue of 39 long tunnels and, at one time, nearly 40 miles of snowsheds, while climbing the up to a 2.5% grade of SP's Sacramento Division via the Donner Pass route. With the cab at the back of the locomotive (normal operation) and exhaust stack in the lead, a number of crews were nearly asphyxiated by the choking exhaust inside these structures. Supplied crew air was tried but not very successful and limited vision and crew mobility. Locomotives were tried run in reverse. This meant that the tender was leading the train and required a pilot to be mounted on the tender. Crew vision was limited by the tender and also put the crewmen on the wrong sides of the cab for seeing railroad signals. Most locos in use at this time on the SP had either no trailing truck or a singular axle trailing truck never designed to be the lead truck on the locomotive, both instances severely limiting train speed. At this time, the SP favored Baldwin Locomotive Works designed locos and contracted them to build a cab-forward designed locomotive, which was found to be so successful the SP ordered more cab forwards before the original prototype had arrived on the SP. Turning the normal locomotive arrangement around now placed the crew well ahead of the exhaust fumes, insulating them from that hazard and from this point onward, all oil-burning articulated locomotives the SP ordered were of the cab forward design. Visibility from the cab was found to be superb with no limitation on what was ahead of the locomotive. The engineer and fireman controls were shifted to the opposite sides of the locomotive cab so that when run "cab ahead" the crew was located on their usual side of the track. Thus the engineer controls were still on the right front side and the fireman controls were on the left side of the cab. Gauges were placed in front of the crew as well so they could look forward out the cab windows and not at the firebox as normal locomotives operate.
There were crew concerns about what would happen to the enginemen in the event of a collision with a gasoline tanker truck at a grade crossing as they would be right on top of it when it exploded. But these concerns were allayed partially because of the unobstructed view from the cab and in the over 45 years of SP Cab Forward operations no tankers were hit.
With the cab forward design, the oil and water tanks were behind the smokebox and the firebox was all the way now at the front of the locomotive, a sizeable distance to push the lifeblood of a steam loco, especially the tar like consistency of Bunker C fuel oil. It was quickly determined that adding a constant 5 psi to these holding tanks in the tender would get both the oil and water to both flow normally to the firebox and boiler, respectively, even on steep uphill grades.
Many railroads had unique locomotives created to fit their needs but all were still just a standard loco design of exhaust in the front, cab in the rear. Cab Forward locomotives became an easily recognizable and unique symbol of the Southern Pacific Railroad.
References:
- Cab-forward: The Story of Southern Pacific Articulated Locomotives by Robert J. Church
- Cab Forward 4294: Southern Pacific Railroad's Signature Locomotive by David N. Anderson, Vincent C. Cipolla, and Russell M. H. O'Day
- Cab-In-Front: The Half Century Story of An Unconventional Locomotive by John B. Hungerford
- A Century of SP Steam Locomotives by Guy L. Dunscomb
- Eddystone 1939: Building A Southern Pacific Cab-Forward At Baldwins by James E. Kranefeld. National Railway Bulletin, Volume 61, Number 4, 1996.
- Memories of the Cab Forwards - A Personal View by Richard Lohse
- Southern Pacific Company Steam Locomotive Compendium by Timothy Diebert and Joseph Strapac
- Southern Pacific Steam Pictorial Vol.2 by Guy Dunscomb, Donald Dunscomb, and Robert Pecotich
- Portrait of an Engine Southern Pacific Cab Forward 4249 by Joseph J. Bothhman
- Those Amazing Cab Forwards by George H. Harlan
- Southern Pacific Steam Series Volume 7 -- Southern Pacific Articulated 4-8-8-2 AC-4 to AC-6 Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 10 -- Southern Pacific Articulated 4-8-8-2 AC-7 and AC-8 Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 13 -- Southern Pacific Articulated 4-8-8-2 AC-10 to AC-12 Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 15 -- Southern Pacific Articulated AC's and MC's Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 19 -- Southern Pacific Misc. Mallets Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 27 -- Southern Pacific AC-1 to AC-3 Class 2-8-8-2, AC-4 to AC- 12 Class 4-8-8-2 Cab Forward Pictorial - by Jeff Ainsworth
- Southern Pacific Steam Series Volume 32 -- Southern Pacific AC-1 to AC-3 Class 2-8-8-2, AC-4 to AC- 12 Class 4-8-8-2 Cab Forward Pictorial - by Jeff Ainsworth
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