Southern Pacific 4294 on display at the California State Railroad Museum.

(Neil916 at English Wikipedia, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons)

 

Southern Pacific herald.

CAB FORWARD LOCOMOTIVES

In steam locomotive design, a cab forward design typically has the cab placed forward of the boiler at the very front of the engine. On what are typically oil-fired locomotives, the fireman's station is in the forward cab alongside the engineer.

Visibility is greatly improved from the cab, and fumes from the smokestack do not fill a forward cab in tunnels. However, the crew's prospects in the event of a collision are worse.

 

"Ariel", a Forney-type cab-forward locomotive: Front is to the left in this image (note location of headlight and cowcatcher).

(See page for author, Public domain, via Wikimedia Commons)

 

Forney design

Matthias N. Forney was issued a patent in the late 1860s for a new locomotive design. He had set out to improve the factor of adhesion by putting as much of the boiler's weight as possible on the driving wheels, omitting the pilot wheels from beneath the front of the boiler. Such a design would not have been stable at high speeds on the rather uneven tracks which were common at the time. Instead, he extended the locomotive frame behind the cab, placing a four-wheel truck beneath the water tank and coal bunker. In conventional Whyte notation, this resulted in a 0-4-4T locomotive, but when run in reverse it was effectively a 4-4-0T, with the track stability of that popular wheel arrangement, along with unobstructed visibility for the engineer, and improved dispersal of smoke and steam.

Forney's design proved ideal for the small, nimble locomotives for elevated and commuter railroads, and he licensed the patent design to many manufacturers. Large numbers of Forneys served in New York City, Boston, Chicago and elsewhere, but were superseded at the end of the nineteenth century by electrification and the development of subways.

Ariel and Puck were 2 ft (610 mm) gauge locomotives built to the Forney cab-forward design for the Billerica and Bedford Railroad in 1877 by Hinkley Locomotive Works of Boston.

 

Mallet compound locomotive, Southern Pacific Railroad.

(Postcard, Public domain, via Wikimedia Commons)

 

Southern Pacific Railroad

The best known example of the cab-forward design in the United States, the Southern Pacific Cab-Forward (also known to a lesser extent as "Cab-in-fronts" and "Cab-aheads") placed the cab at the front by the simple expedient of turning the entire locomotive, minus the tender, by 180 degrees. This arrangement was made possible by burning fuel oil instead of coal.

The cab forward design was widely used by the Southern Pacific Railroad. The design was able to deal with the peculiar problems of its routes. The 39 long tunnels and nearly 40 miles (64 km) of snow sheds of the Sierra Nevada could funnel dangerous exhaust fumes back into the crew compartment of a conventional locomotive. After a number of crews nearly asphyxiated, the locomotive was run in reverse. This meant that the tender was leading the train, which introduced new problems. The tender blocked the view ahead and put crewmen on the wrong sides of the cab for seeing signals. The tenders were not designed to be pushed at the lead of the train, which limited speeds. Southern Pacific commissioned Baldwin Locomotive Works to build a prototype cab-forward locomotive, then ordered more units before the prototype had even arrived.

All of the cab-forwards were oil-burning locomotives, which meant there was little trouble involved putting the tender at what would normally be the front of the locomotive. The oil and water tanks were pressurized so that both would flow normally even on uphill grades. Visibility from the cab was superb, such that one crewman could easily survey both sides of the track. There were concerns about what would happen to the crew in the event of a collision, and at least one fatal accident occurred on the Modoc Line in Herlong, California when a moving locomotive struck a flat car. Turning the normal locomotive arrangement around also placed the crew well ahead of the exhaust fumes, insulating them from that hazard. One problematic aspect of the design, however, was the routing of the oil lines; because the firebox was located ahead of the driving wheels (instead of behind them, the usual practice), oil leaks could cause the wheels to slip. A nuisance under most conditions, it sadly resulted in at least one fatal accident. This occurred in 1941 when a cab-forward with leaking steam entered the tunnel at Santa Susana Pass, near Los Angeles. The tunnel was on a grade, and as the slow-moving train ascended the tunnel, water on the rails from a leaking cylinder cock caused the wheels to slip and spin. The train slipped backward and a coupler knuckle broke, separating the air line, causing an emergency brake application and stalling the train in a tunnel that was rapidly filling with exhaust fumes and steam. The oil dripping on the ties then ignited beneath the cab, killing the crew.

No other North American railroad ordered cab-forward locomotives, although some, like Western Pacific, did consider the type. Built to deal with difficult terrain, these locomotives became an easily recognizable symbol of the Southern Pacific. In total 256 such Mallet-type articulated locomotives, in three different wheel arrangements, were placed on SP's roster. One example of the type, Southern Pacific 4294, is kept at the California State Railroad Museum in Sacramento, California. It is a 4-8-8-2 locomotive and is the only one to escape being scrapped. It was also SP's last new steam locomotive, built in 1944.

Cab forward steam locomotives of Southern Pacific Railroad

2-6-6-2

MM-2

4-6-6-2

AM-2

2-8-8-2
MC-1

MC-2

AC-1

MC-4

MC-6

AC-3

 

4-8-8-2

AC-4

AC-5

AC-6

AC-7

AC-8

AC-10

AC -11

AC-12

 

Preserved
SP 4294 (AC-12)

North Pacific Coast locomotive 21, an early cab-forward experiment.

("Trains" magazine, February 1956, page 44, Public domain, via Wikimedia Commons.)

 

North Pacific Coast Railroad

A decade before SP's first cab forward, the North Pacific Coast Railroad, later part of the SP-owned Northwestern Pacific company, rebuilt an 1875 4-4-0 into an oil-fired cab-forward locomotive. This innovative engine was built by William (Bill) Thomas, the NPC master mechanic who was nationally known and holder of a number of patents. Thomas used the running gear and frame from NPC locomotive 5, the "Bodega", which had been wrecked in 1897, to build NPC 21. With the addition a new and unusual marine water tube boiler and an all-steel cab, installed in reverse order from standard engines, this unique creation earned Thomas a patent on the locomotive design. No. 21 entered service in 1900, but only lasted a few years. Although it reportedly steamed well, though with a sooty exhaust, the crews found it difficult to operate, and with fears of the possible results of a collision they dubbed it "The Freak". A negligent fireman allowed the water level to drop, damaging the boiler, and it was not repaired.