The Southern Pacific Cab Forward

Cab Forwards were a distinct trademark of the Southern Pacific. They were sometimes also called "Cab-in-fronts" or "Backup Mallies" (even though, technically, only some of the first classes were true mallets). According to the diffinitive book on Cab Forwards (Those Amazing Cab Forwards by George Harlan), no other railroad in the world had locomotives like them. However, a few other unique examples did exist.

Why Cab Forwards?

The Sacramento division of the Southern Pacific had close to 150 miles of grades of up to 2.5%. On this Roseville - Sparks line over the Sierra Nevada there were also almost 30 miles of snow sheds and tunnels. Over the years, as trains grew in length, more powerful locomotives were required.

In 1908 the Southern Pacific ordered two 2-8-8-2 mallets classified MC-1 (Mallet - Consolidation) numbered 4000 and 4001. On a trial run up the "Hill" two problems became immediately evident. First, the great volume of exhaust gasses almost asphyxiated the crew. Second, the stack exhaust velocity was so great that it blew the roof boards off of the snow sheds. The second problem was easily handled by installing "stack splitters" (a deflector located above the smoke stack which directed the exhaust to the sides), as shown at the top of the image on the right. The first problem required more consideration.

Shortly after delivery of the MC-1s, an enterprising engineer decided not put up with nearly being asphyxiated or exposing himself to the tremendous heat and noise. He had the engine turned, hooked the engine pilot to the front of the train, and backed his locomotive over the hill pulling the train behind. This alleviated the above problems but created others such as pushing the tender ahead of the engine and the engineer being on wrong side for the signals. Other engineers began following this example.

A team of Southern Pacific design engineers came up with a plan and designs for a mallet with the cab in front, classified MC-2. Southern Pacific had Baldwin build 15 without testing one! Numbered 4002-4016, they were delivered in February and March of 1910. The engineer's and fireman's controls were shifted to opposite sides of the cab so that when run "backwards" the crew was on the usual side of the track.

Since the firebox on these locomotive was located in the front (far from the tender), they were designed to burn oil. Oil was piped from the tender along the locomotive to the firebox. The oil bunker in the tender on these locomotives was made air-tight and was structurally braced. They were slightly pressurized with air from the main air reservoir to insure a constant oil flow to the burner in the fire box when to the locomotive when traveling upgrade.

Cab After the MC-2s had proven themselves, 32 more, classified MC-4 and MC-6 (MC-3 and MC-5 were skipped) were ordered. Before it was all over, Southern Pacific ended up with a total of 256 Cab Forwards (all classes).

Although the crews initially complained about concerns that if they hit a gasoline truck at a grade crossing they would be right on top of it when it exploded. Fortunately, in 46 years of running Cab Forwards, this never happened. This was partially because of the unobstructed view from the cab. The advantage in visibility was tremendous.

Cab Markings

What did those Cab Forward cab markings mean?

		24	24
AC-4	AC63			475 SF
		32

AC-4	Class designation
AC63	Articulated Consolidation, 63 inch drivers
24 24	Cylinder diameter (front and rear) in inches
32	Piston stroke in inches
475	Weight on drivers in thousands of pounds
SF	Superheated, Feedwater heater

Southern Pacific Yellowstones

Not all Southern Pacific articulateds were Cab Forwards. Class AC-9 was made up of 12 Lima built Superpower, coal burning Yellowstones (2-8-8-4). They had the same tractive effort of the Cab Forwards. They were numbered 3800 - 3811 and were used in the Southwest.

Cab Forward Classes

Class	Locomotive Number	Wheel Arrangement	Driver Diameter	Cylinders	Steam Pressure	Tractive Effort	Horse Power
MC-1	4000-4001	2-8-8-2	57	26x40x30	200	85,040	4000
MC-2	4002-4016	2-8-8-2	57	26x40x30	200	85,040	4000
AC-1	4000-4016	2-8-8-2	57	22x22x30	210	90,940	4350
MC-4	4017-4028	2-8-8-2	57	26x40x30	200	85,040	4000
AC-2	4017-4028	2-8-8-2	57	22x22x30	210	90,940	4350
MC-6	4029-4048	2-8-8-2	57	26x40x30	200	85,040	4000
AC-3	4029-4048	2-8-8-2	57	22x22x30	210	90,940	4350
MM-2	4200-4211	4-6-6-2	63	25x38x28	200	65,900	3000
AM-2	3900-3911	4-6-6-2	63	22x22x28	210	76,800	3450
AC-4	4100-4109	4-8-8-2	63	24x24x32	235	116,900	5640
AC-5	4110-4125	4-8-8-2	63	24x24x32	235	116,900	5640
AC-6	4126-4150	4-8-8-2	63	24x24x32	250	124,300	6000
AC-7	4151-4176	4-8-8-2	63	24x24x32	250	124,300	6000
AC-8	4177-4204	4-8-8-2	63	24x24x32	250	124,300	6000
AC-9*	3800-3811	2-8-8-4	63	24x24x32	250	124,300	6000
AC-10	4205-4244	4-8-8-2	63	24x24x32	250	124,300	6000
AC-11	4245-4274	4-8-8-2	63	24x24x32	250	124,300	6000
AC-12	4275-4294	4-8-8-2	63	24x24x32	250	124,300	6000

NOTE: Class AC-9 were Yellowstones (not Cab Forwards)

The Monkey Deck

The Southern Pacific Cab Forwards had a platform immediately following the smokebox of the locomotive (as shown in the figure on the right). This platform was called the "monkey deck. Smoke boxes were often stained with rust from boiler water and boiler water treatment chemicals. Because of its proximity to the stacks, the monkey deck was often stained with rust from boiler water too. Occasionally articulateds would spew hot water and mud from their stacks. Most railroad employees were aware of this fact. However, many hoboes thought the monkey deck was a good place to ride. After passing through tunnels or snow sheds, hoboes riding the deck were either scalded or asphyxiated depending upon how the locomotive was performing. The monkey deck was not a good place to ride on Cab Forward locomotives.

The Last Cab Forwards

In November, 1957 4274 was brought out of retirement to make the last run of a Cab Forward over Donner Pass.
In 1960 the last Cab Forward to be scrapped was AC-10 4243.
On May 7, 1981 AC-12 4294 was designated a National Historic Mechanical Engineering Landmark.
4274 was the last Cab Forward to travel over Donner Pass.

Cab Forward Photos

AC-6 4150 (Photo courtesy Byron Bostwick
AC-8 4198 (Photo courtesy Byron Bostwick
AC-10 4206 (Roseville, CA, Walter R. Evans photo)
AC-10 4216 (Roseville roundhouse turntable, CA, Joe Dixon photo)
AC-11 4247
AC-12 4285 (Photo courtesy Byron Bostwick