Modeling SP Road Engines (Part 4) - Articulated Steam

I'll be covering Modeling SP Articulated Road engines which unlike the common belief, were not only used on Donner Pass, but the SP used all around the system over the years.

SP 4255, an AC-11, swings into the curve at Caliente, a great example of SP's heavy steam engines.

In the last three blog posts in this topic I've covered the Modeling SP Road Switchers (Part 1) - Small Steam, Modeling SP Road Switchers (Part 2) - Medium Steam, and Modeling SP Heavy Road Steam Engines.  Next time we'll be wrapping up with the SP's Articulated Steam Engines, followed by various Passenger Steam Engine types in use during the 1940s and 1950s.

Many thanks to James Salkeld, Brian Black's uncle Nolan Black photographs (unless noted), and Eddie Sims for the use of their collections in putting together this post with the following amazing historical photographs.

Articulated Heavy Steam Engines

MC-1/2/4/6 Classes (2-8-8-2) - Compound Articulated

The Mallet-Consolidation or MC-class engines were built with 57" drivers with the Mallet Compound design using the high pressure steam from the boiler first in the fixed cylinder set under the middle of the boiler, then reusing the steam in the low pressure cylinders, which were much larger, on the radial engine,  beyond the smokebox.  These new engines could haul more tonnage than a standard Consolidation, more efficiently and with only one crew.

SP 4001, the second of two 2-8-8-2, as originally delivered with cab-in-rear, circa 1910. Eddie Sims Collection

The first two engines, 4000 and 4001 were delivered in 1910 and during their first experimental trips over Donner Pass determined to be unsuitable in their as-built configuration.  After a more successful trip made in reverse, the 15 engines of the MC-2 class, which was already ordered and being built at Baldwin were ordered to be changed to cab-forward configuration.  As the new oil burning engines were turned around to put the crew at the front, and the exhaust stack in the rear drawbarred to the tender.

SP 4007 as Mallet Compound, as noted on the photograph.  Eddie Sims Collection.

The oil bunker required the addition of 5 lbs air pressure to be added to the tank to help push the heated heavy fuel oil the extra 50 or so feet to the forward location of the firebox and burner.  An exhaust splitter was also installed on the engines to help prevent damage to the timber snow shed roofs.

SP continued to order more MC-class engines from Baldwin in the cab-forward configuration.
MC-2 SP 4002-4016 (15 engines)
MC-4 SP 4017-4028 (12 engines)
MC-6 SP 4029-4048 (20 engines)

The MC-1 class (4000 & 4001) continued experimenting with air masks for the crews and trying to find solutions for keeping the crews breathing when operating over Donner for most of the year.  In February and March 1910 the new MC-2s arrived and were immediately sent up to work between Roseville and Truckee over Donner Pass.  The MC-1s were transferred to work over the tunnel-less Beaumont Hill east of Colton until 1923, when both the 4000 and 4001 were turned around and rebuilt to MC-2 standards.

SP 4001 after being rebuilt to AC-class after 1928.  Eddie Sims Collection

All the rest of the type was delivered in a "Cab-Forward" configuration.  All of the MC-class enges were rebuilt into the AC-1/2/3 class engines starting in 1928.

MM-1 Class (2-6-6-2) Compound Articulated

The MM-1 Class engines were built for the T&NO as their 950-961, renumbered as the 900-911.  These 12 engines were broken down and the frames reused in 1929-1930 and their parts reused on the heavy M-21 class 2-6-0 engines.  Note, these were never cab-forward style engines. - Note: The M-21s went to the Pacific Lines for a time between 1940 and 1942, but returned to Texas, being scrapped finally in the early 1950s, not being light enough on-drivers to be sent out on the branchlines, and made obsolete earlier than other 2-6-0s because they weighed nearly as much as a standard C-class 2-8-0.

MM-2 Class (2-6-6-2 & 4-6-6-2) - Compound Articulated

The MM-series of engines started after the first MC-class engines were delivered and SP decided they needed a 'passenger version', so they designed a Mallet-Mogul.   Originally built in 1911 as the 4200-class the twelve engines were renumbered to the 3900-series (3900-3911) to clear the 4200-series for the new AC-8/10/11/12 class engines in the late 1930s.

This type was plagued with several high profile derailments and roll-overs on passenger assignments.  It was determined that the type's two-wheel truck under the cab was not sufficient to properly lead the engine into a curve at speed, as it was still based on a two-wheel trailing truck design!

I wasn't able to get permission to use a pre-rebuild photo of an MM before rebuilding.  Eddie Sims Collection.

The solution came with the conversion to a 4-wheel pilot truck design, similar to other engines such as the 4-6-0 and 4-4-2 wheel arrangements used.  All twelve engines of the type were rebuilt to 4-6-6-2 configuration.

MM-3 Class (2-6-6-2) - Compound Articulated

SP 3930 and 3931 ex-Verde Tunnel & Smelter RR engines, acquired by the SP in 1943.  They were 'normal' configuration articulated 2-6-6-2s, very similar to the WP's 2-6-6-2s, but never converted to 'simple' operation, remaining compounds until the end.

(Sorry, no photo permission available yet, but here's a link to two photos of SP 3930 & SP 3931)

The two engines worked primarily in drag transfer service between Taylor Yard in LA and the yards in Colton (remember this is before 1967's new West Colton Yard complex).  The engines lasted until 1954 and 1951 respectively.

AC-1/2/3 Class (2-8-8-2) - Simple Articulated

SP 4008 near the end of her life as seen after June 1946. - Eddie Sims Collection

The AC-1/2/3 class engines were rebuilt from the MC-1/2/4/6 class compound engines starting in the late 1920s.  The rebuild program was started after the new AC-4s showed that a simple (non-compound) articulated engine with super heater and feed water heater could out perform the new SP-1/2/3 class engines in pulling power and speed, which breathed new life into the older articulated engines.

AC-1 (ex-MC-1) 4000-4001
AC-1 (ex-MC-2) 4002-4016
AC-2 (ex-MC-4) 4017-4028
AC-3 (ex-MC-6) 4029-4048

SP 4020 in the last few years of her life with large SP lettering on the tender side. - Eddie Sims Collection

One of the main visual differences of the rebuilt AC-1/2/3 class engines was the exterior steam pipes now came from the smoke box, with super heated steam instead of out of the steam dome to the fixed engine cylinders.

WSM's SP AC-3 class 2-8-8-2 with 120-SC-class tender.  JHill Photo

For the HO modeler in brass, Westside Models and I believe Key have imported the AC-1/2/3 class rebuilt engines.  Most recently TCY imported the early 2-8-8-2 compound versions around 2016-2017.

AM-2 (4-6-6-2) - Simple Articulated

Here's a couple of photos of the 3900-class after being rebuilt with a 4-wheel pilot truck and simplified (removing of the compound cylinder operation) on the engines.

SP 3902 with a 'whale back' tender during the last few years of life.  Eddie Sims.

 The type continued in service until the late 1940's, during WWII primarily working in Oregon.

Here we see a pair of eastbound AMs climbing Altamont Pass, west of Tracy. Eddie Sims Collection

AC-4/5/6 Class (4-8-8-2) - Simple Articulated

SP 4109, AC-4 class in a rare color photo near the end. - Eddie Sims Collection

The SP decided in 1928 that they should see about improving the design of the MC-6, MM-2, and bring it up to par with the SP-1/2/3 class 4-10-2s and F-5 class 2-10-2s for freight service.  The resulting design used 63" drivers of the SP's large dual service freight and passenger engines, the 4-wheel pilot truck for higher speed operations and proper support of the cab while running "backwards" compared to conventional designs.  The new engines were superheated and also designed to use feed water heaters for maximum efficiency and economy of fuel and water used.  The class would also use the large 16000 gallon 'water bottom' tenders, the largest Vanderbilt-patent tenders used on the SP, with the same 5 PSI applied to the fuel bunker, as on the MC and MM type engines.

The new type was assigned the 4100-series.
AC-4 4100-4109 (10 engines)
AC-5 4110-4125 (15 engines)
AC-6 4126-4150 (25 engines)

SP 4112 arriving with No.60, the West Coast from Portland in 1940 at LA River Station. - Eddie Sims Collection

The new and bigger AC-class engines were used in passenger service, replacing the older Mk-5/6, SP, and F-class engines in heavy mountain passenger service.  As the later AC series of engines  (AC-7 and above) with disc drivers were arriving, the earlier engines with spoked drivers (AC-4/5/6) were assigned to freight service only.

SP 4143 an AC-6 engine rests between trips after 1946. - Eddie Sims Collection

A couple of the earlier AC-4/5/6 class engines were refitted over the years with later AC-8 type cabs either, usually as a result of wrecks.  In one case the engine (an AC-3, iirc) that received the AC-8 cab was in the shop when another engine was involved in a wreck.  The wrecked engine took the original cab from the engine in the shop, which had been removed for boiler work, after whichtime the shopped engine was given a newly fabricated cab.

SP 4146 with rebuild AC-8-type cab works Extra with a short reefer block. - Eddie Sims Collection

The early AC-4/5/6 class engines worked into the late 1940s and the last few survived on the flatlands of the Western Division until the early 1950s.  The most engines of the AC-4/5 classes being retired by around 1953.  Some of the AC-6's lasted a bit longer, but even the later AC-class engines were being retired in the 1953-1954 timeframe.  The SP 4149 lasting the longest, being vacated finally on December 16th, 1955 at Sacramento.

Models of AC-4/5 class engines have been produced in brass by several manufactures, including WSM/KTM.  The AC4/5s also have been made in plastic by BLI.  Precision Craft produced a run of AC-6s as a hybrid model about 10 years ago.

AC-7/8/10/11/12 Class (4-8-8-2) - Simple Articulated

A filthy SP 4157 is seen here with her pressed steel pilot with plow wings. - Eddie Sims Collection

The AC-7 class engines can be recognized by the pressed steel (corrugated) sheet metal pilot, where as the AC-8 class engines had the later smoother bulbous plow-pilot like the later AC-10/11/12 class engines.

AC-7 4151-4176 (26 engines)
AC-8 4177-4204 (28 engines)
AC-10 4205-4244 (40 engines)
AC-11 4245-4274 (30 engines)
AC-12 4275-4294 (20 engines)

Westbound SP 4198 blasts out of Tunnel 18 in Soledad Canyon with First 803 after 1946. - Eddie Sims Collection

Here we see a couple of AC-8 working passenger service over Tehachapi Pass.  They also worked passenger service over the Cascades and Donner Pass on the heavier (non-streamlined) trains.

SP 4199 leads a string of headend baggage and RPO cars in Soledad Canyon - Eddie Sims Collection.

As we see in these photos, the later AC-class engines were used in passenger service, but were limited with a system wide ETT (Employee Timetable) speed limit of 60 MPH with train.  This restriction kept them off of the premier train assignments outside of heavy mountain divisions with lower speed limits, such as Donner, Tehcahapi, Shasta/Cascade.

SP 4230 leads No.60, the West Coast, from Sacramento via Bakersfield at Newhall, 4-29-51. - Brian Black Collection. used with permission

The AC-class engines were commonly assigned to trains such as the Pacific Limited, Overland, Owl, Klamath, West Coast, Beaver, and many other secondary trains in mountain territory until the arrival of dual-service F-units around 1952-53.  A good example of a train that they were not regularly used on is the regular San Joaquin Daylight, which required 70 MPH across the Antelope Valley, between Mojave and Palmdale to keep up with the schedule.  - Of course if the regular road engine for a premier passenger train failed on the road so badly that they couldn't get the train into a station with protection passenger engines, then the SP would have used any engine that could pull the crippled train in.

SP 4258 leads the Owl (No.58) across the LA river into LAUPT. - John Shaw Photo, James Salkeld Collection, used with permission

SP 4283 pulls PRS Excursion April 1949 at Rosamond running as First 803. - Nolan Black Photo, Brian Black Collection, used with permission

The AC's continued to work in freight service on the mountain divisions of the SP until replaced in the early 1950s by F-units.  The large groups of 6-axle diesel road switchers arrived in 1954 displaced most the remaining AC's off mountain territories and onto the flatter San Joaquin Valley and Western Divisions until the end in 1956 when more diesels arrived.  Heavy steam only very occasional operated in 1957, and the order to cut all the fires in late 1958.

SP 4279 works upgrade into East Mojave with Third 803 past the east end of the yard. - Eddie Sims Collection

A Quick Note About Tender Weathering

The AC's tenders sometimes had an interesting weathering pattern that showed up when the water was cold, and the air was humid.  Condensation would form below the water level in the tender.  As seen in the photo below, the sheen difference of the dry upper cistern side and the wet lower side.  Also notice that the oil bunker often is shinier because it is heated, allowing the oil to flow to the burner, and doesn't get the dust sticking to it.  The dirt however quickly clings to the condensation on the cistern sides, resulting in a dusty and dirty appearance.

The SP 4292 shows a common weathering condition on the tender in this photo. - Eddie Sims Collection

This is one of the things that is lost if you base your weathering on an engine which is 'dead' and not under steam.  Heat changes the sheen of the metal, the crew's care of the engines lubrication will look different than an out-of-service engine, and a 'dead' engine might have been weeks or months since it was last given a once-over with even 'running repairs' between trips.

The Last Survivor...

Unfortunately of all the AC's that the SP offered to various towns and cities, there was only one saved from the scrapper's torch.

SP 4294 at Watsonville Jct in 1955. Eddie Sims Collection

The last AC-12 built, the 4294 still exists today and is on display at the California State Railroad Museum in Sacramento, CA.

Models of Late AC-class Engines

Several companies have imported later style AC's in HO brass; WSM/KTM, Balboa/KTM, and Challenger Imports just to name a few.  Key's are ok, but generally don't have as robust of drives as the KTM built engines of the WSM and Balboa imported engines.

Akane made late AC-class engines, but I steer clear of them because of the fiber insulation on the drivers, which often is failing these days on second hand engines, and the drive train is not a nice straight line shaft, as on the KTM engines, instead having a kink between the motor and the front engine gear box.  Given the overall detail and proportions of the models, I don't see much use for them.

Sunset Models, PFM, and Tenshodo also made late model AC-class engines.  I've personally only tinkered some with a Sunset Models AC-10, which I may do a post about at some point, currently I feel that they're not quite as good as the 1990s Sunset F-class 2-10-2s.

In plastic Rivarossi made a model, which is pretty much obsolete today.  IMRC produced a plastic model of an AC-12 around 2007, and has worked on the various mechanical problems over the following two runs of models.  IMRC has also expanded the versions to include an AC-8 with small changes to the front end details around the cab.

I cover my 10 Years of Dealing with the IMRC AC-12's issues and some repairs that I did on a friend's AC-12 (3rd run) model on my blog about IMRC Mk-3 AC-10 Damage Control.

AC-9 Class (2-8-8-4) - Simple Articulated (Cab-in-Rear)

SP 3805 with excess width warning stenciling on the front cylinders. - Eddie Sims Collection

The only "proper" Yellowstone (2-8-8-4) arrangement engines on the SP were the twelve AC-9 class engines built for use on the Rio Grande Division in the New Mexico and Arizona Deserts as coal burners.  They earned the nickname "Gila Monster" because of their banishment to live in the deserts.

AC-9 3800-3811 (12 engines)

SP 3800 working its way west at Mojave on the way to the Modoc Line.

After the closure of the Dawson Coal Mines, they were converted to burn oil, received new second hand tenders, and were transferred to the Modoc Line from Klamath Falls to Alturas and connecting to the Overland Route deep in the Nevada Deserts.

The unique AC-9s have been imported in HO brass over the years, I believe either Key or Balboa and WSM/KTM imported them.  Akane also imported one, but watch out for the drive line and driver insulation on those early engines!

The End

Markers into the sunset.

In the next blog post on this general topic, I'll be opening the throttle to cover Modeling SP's Passenger Steam Engines of the late 1940s and 1950s.

Jason Hill

Related Articles:

Modeling SP' Road Switchers (Part 1) - Light Steam Engines

Modeling SP's Road Switchers (Part 2) - Medium Steam Engines

Modeling SP's Road Engines (Part 3) - Heavy Steam

Modeling SP Road Engines (Part 4) - Articulated Engines

Modeling SP Road Engines (Part 5) - Passenger Steam

Busy Times in Bakersfield (Part 1) - Operations Around the Roundhouse

Locomotives of the Southern Pacific - (Spotters Guide from the age of steam!)