Steam Locomotive Development in North America

Steam Locomotive Development in North America

Last time we explored some practical model railroad issues. If
you’ve looked at model steam locomotives you probably noticed the many
different kinds. But why were there all those different types of
locomotives? So let’s look at locomotive evolution on the real
railroads.

How Do We Describe Steam Locomotives?

Before we can talk intelligently about the evolution of steam
locomotives, we need to discuss how to describe or classify them. While
there are different ways to do that, for the common rod locomotives,
the most useful, and thus the most common, is called the Whyte system.

The Whyte system is fairly simple. Starting at the front of the
locomotive, count the number of non-powered wheels in the front, or
pilot, truck if any. Then count the number of driving wheels. Finally,
count the number of non-powered wheels, if any, in the trailing truck
behind the driving wheels. For example, a locomotive with four wheels
in the pilot truck, six driving wheels, and two wheels in a trailing
truck behind the drivers, would be described as a 4-6-2. A locomotive
with two wheels on its pilot truck, eight drivers, and four wheels in
its trailing truck would be described as a 2-8-4.

In addition to the wheel arrangement, other issues arise. The
simplest is whether or not the locomotive has a separate tender for
fuel and water. If not, there are water tanks and separate fuel tanks
or bunkers on the locomotive; this is indicated by a “T” after the
wheel arrangement to indicate tank locomotive, meaning no
separate tender. An example is a 2-8-2T logging locomotive. Tank
locomotives did not have the long range of locomotives with separate
tenders, which carried more fuel and water, and were therefore used in
short-range operations such as mining, logging, switching, and commuter
service.

The above discussion concerns the rod locomotive, the most common
design of steam locomotive, in which the driving wheels are moved by
rods connected to the steam pistons. However, small, specialized
locomotives used gear drive instead of rod drive. These were commonly
used for track with sharp curves or steep slopes, such as are found in
logging or mining operations. The most common geared locomotive designs
are the Shay, Heisler, and Climax, named for their inventors or
manufacturers. These are nearly all tank locomotives and are described
by their names rather than a Whyte designation since all wheels are
driven.

There were many different wheel configurations, and the most popular
were even named for convenience when talking about them. But why were
there so many? One reason was that railroads ordered steam locomotives
custom-built to their specifications, so the variety of steam
locomotives was greater than Diesels, which are far more standardized
by their manufacturers. But that does not explain all the variations.
Steam locomotives were built for different purposes; switching duties,
freight hauling, and passenger hauling. Also, locomotive types evolved
as operating requirements changed and technology improved.

Switching Steamers

Switch locomotives generally needed to be compact to operate
effectively in freight yards. They also needed to have lots of pulling
power, called tractive effort. To achieve these conflicting
requirements, most had all their weight on their driving wheels, and
did not use pilot trucks or trailing trucks. Pilot trucks help
locomotives stay on the tracks, but because switch locomotives work at
low speeds, they were able to operate safely without pilot trucks.
Small switchers had four drivers, medium switchers had six drivers, and
large switchers had eight or even ten drivers. These types are written:

0-4-0 0-6-0 0-8-0 0-10-0

The medium 0-6-0 was most common, followed by the 0-8-0. In addition
to the locomotives specifically designed for switching duties, older,
smaller freight locomotives were also commonly used for switching. This
enabled the railroads to avoid buying specially designed switch
locomotives and get extra use from their obsolescent small freight
locomotives. For example, older 2-8-0 freight locomotives were commonly
used for switching, as were, to a lesser extent, 2-6-0 and 2-8-2
locomotives.

Freight Steamers

Steam locomotives designed to haul freight initially had 2-wheel
pilot trucks to keep as much weight on the drivers as possible for
maximum tractive effort. Due to slow freight speeds, 2-wheel pilot
trucks were generally sufficient; 4-wheel pilot trucks were not usually
necessary. However, after WWI freight train speeds had to be increased
due to competition from trucks, and 4-wheel pilot trucks became more
common on freight locomotives for better tracking at those higher
speeds. In addition, the 4-10-2 and 4-12-2 type freight locomotives
were 3-cylinder designs and needed the larger 4-wheel pilot truck to
support the additional weight of a third, center piston and cylinder.

At first, trailing trucks were not needed. However, locomotives
became larger and more powerful as the need to haul larger, heavier
trains increased. Eventually the firebox on some locomotives became too
large to fit in the space between the aft wheels and was often extended
behind them. Two-wheel trailing trucks were needed to support the
greater weight of the enlarged firebox that often extended behind the
driving wheels. In addition, the Santa Fe railroad faced a situation
where, in one mountainous location, their 2-10-0 helper locomotives had
to back down the mountain for a long distance. To help them move
backwards faster without derailing, a 2-wheel trailing truck was added
to create a 2-10-2. So while the trailing truck came into use primarily
to support larger fireboxes, it also enabled locomotives to run
backwards safely at higher speeds.

Initially, the tractive effort of steam locomotives, their pulling
power, was the most important criteria because it determined the size
of the train they could start and pull. After WWI the need for greater
power to move trains faster resulted in a number of innovations, and in
particular a new locomotive type with much larger firebox for more
power. This larger firebox required a larger, 4-wheel trailing truck
for support, and the 2-8-4 type was developed. This was the first of
the so-called super-power locomotives, and introduced the final
development stage in steam locomotives that was soon followed by the
2-10-4, 4-6-4, 4-8-4, and modern articulateds.

Another path of development led to large articulated locomotives.
These locomotives had two sets of drivers supporting a single boiler.
To enable these huge locomotives to go around curves, the forward set
of drivers was articulated, or allowed to turn with respect to the rear
set of drivers, that were built solid with the firebox and frame of the
locomotive. A typical articulated locomotive would be designated
2-8-8-2, which means the locomotive had a 2-wheel pilot truck, two sets
of eight drivers, and a 2-wheel trailing truck.

Technology also improved as engineers worked to improve safety and
efficiency. In addition to tractive effort and power, fuel efficiency
was critical and two important technical developments were applied in
the 1900–1920 timeframe to improve it. First was the feedwater heater,
a device to heat the cold water before it was injected into the boiler.
The feedwater heater reduced the fuel needed to heat the cold feedwater
by heating the feedwater using waste heat before it was injected into
the boiler. This also helped the fireman maintain proper steam pressure
and water level in the boiler when adding feedwater. The second device
was the superheater, which heated the steam to higher temperature than
was needed just to produce the steam. The hotter steam was
thermodynamically more efficient and also reduced the problem of steam
condensing to water after it contacted the pistons. Not only were the
feedwater heater and superheater applied to new locomotives, they were
also retrofitted to many existing locomotives to increase their fuel
efficiency.

Yet another technique applied to steam locomotives to improve their
fuel efficiency was compounding. Compounding means running the steam
through the pistons more than once. So-called simple steam engines run
the steam through the piston once, then it is exhausted to the air. In
contrast, a compound steam engine runs the steam at high pressure to
the first piston; the exhaust of that high-pressure piston is then run
to a larger low-pressure piston to extract additional work from the
same steam. While theoretically more fuel efficient, compounding
generally resulted in slow-speed locomotives and required much more
maintenance than a simple steam locomotive – a very serious problem
given that simple steam locomotives already required considerable
maintenance. Although compounding was used on a number of steam
locomotives, many, perhaps most, compound steam locomotives were
eventually converted to simple steam locomotives without loss of
efficiency and with a gain in speed and maintainability. Except for a
few experiments, no simple steam locomotives were converted to complex
locomotives.

The general evolution of freight steam locomotive types by
wheel arrangement is shown below, with the arrows indicating the
direction of development. For example, the 2-10-4 was an enlarged 2-8-4
and was not, as one might suppose, developed from the 2-10-2, which was
an earlier design. Although the first 2-10-4 was actually a 2-10-2
modified to have a 4-wheel trailing truck, this was a one-of-a-kind
prototype that was not developed further. All other 2-10-4 locomotives
were developed from the 2-8-4 type, which was a ‘super-power’
locomotive with larger firebox.

2-6-0 ? 2-6-2

      ?

2-8-0 ? 2-8-2 ? 2-10-2 ? 4-10-2 ? 4-12-2 ? 4-6-6-4 ?
4-8-8-4

      ? ? (articulated) (articulated)

2-10-0  2-8-4 ? 2-10-4

Articulated Freight Steamers

0-8-8-0 ? 2-8-8-0 ? 2-8-8-2 ? 2-8-8-4

            ?

      4-8-8-2 (Cab Forward)

Passenger Steamers

Passenger steam locos usually had 4-wheel pilot trucks for better
tracking at the higher speeds used by passenger trains. Furthermore,
their driving wheels were larger diameter than those of freight
locomotives to enable higher speeds to be reached. The general development of steam locomotives designed for passenger service is shown below.

4-4-0 ? 4-4-2  ? 4-4-4

      ? ?

4-6-0 ? 4-6-2 ? 4-8-2 ? 4-8-4

            ?

            4-6-4

The 4-6-4 was used for high-speed passenger service on routes with
shorter trains or that were nearly level. In the West the passenger
trains were fewer and longer than in the East, requiring more powerful
locomotives to handle larger trains over mountains. The larger, more
powerful 4-8-4 became the ultimate passenger steamer and was also
powerful enough for use as a fast freight locomotive.

Although early articulated locomotives were usually too slow for
passenger service, there were at least two notable exceptions among
modern articulated locomotives. The Union Pacific Railroad designed
their Challenger type 4-6-6-4 for dual service, and this type was so
successful it was purchased by a number of other railroads. And newer
versions of the Southern Pacific’s unique Cab Forward articulated
locomotives had 4-wheel pilot trucks and were capable of the higher
speeds required by passenger trains. The mighty Cab Forwards were used
in passenger service over mountain routes.

In addition to the dual-service articulated locomotives noted above,
some other locomotives, mostly 4-8-4 types, were designed with both
freight and passenger service in mind. Finally, passenger trains were
often pulled by steam locos designed for freight service when passenger
locomotives were not available.

Southern Pacific’s Unique Cab Forwards

Due to the extensive length of snow tunnels on the Southern
Pacific’s difficult route over the Sierra Nevada Mountains (Donner
Pass), the SP found that the smoke and fumes from the large 2-8-8-2
articulated locomotives they attempted to use over this route
asphyxiated the locomotive’s crew. The unorthodox solution was to run
the locomotive in reverse; the crew went through the tunnel before the
smokestack and thus were able to breathe. This was possible because the
SP had already moved to heavy bunker oil instead of coal to power most
of their locomotives. SP then ordered a very successful series of Cab
Forward locomotives that eventually totaled 256 engines. There were a
number of series and types as described below.

Class AM-2: Ordered for fast passenger service, these 2-6-6-2
locomotives were quickly converted to 4-6-6-2 configuration for better
tracking after derailments due to their 2-wheel pilot trucks early in
their career. Originally MM-2 compound locomotives, they were converted
to simple locomotives and re-designated class AM-2. Used as freight locomotives for most of their lives, they were retired in 1948.

Classes AC-1, -2, and -3: Slow but powerful 2-8-8-2 compound
locomotives ordered for freight service, they were originally
designated classes MC-4 and MC-6 but were re-designated classes AC-1,
-2, and –3 when converted to simple locomotives. Despite their slow
speed, these were often used for passenger service over the steep,
difficult Donner Pass, where no locomotive was very fast. Three
production batches of essentially identical locomotives; the last were
retired in 1948. All subsequent AC classes were designed and built as
simple locomotives for higher speed and reduced maintenance.

Classes AC-4, -5, and -6: Powerful, fairly fast 4-8-8-2
locomotives ordered for fast freight service, they were distinctly more
modern than the earlier AC classes in all respects. The AC-6 batch
differed in steam pressure and minor details from the AC-4 and AC-5
batches. Much faster than earlier Cab Forwards and used in passenger
service in the mountains, all lasted until the end of steam.

Classes AC-7, -8, 10, 11, and -12: Powerful, fairly fast
4-8-8-2 locomotives ordered for fast freight service and also used in
passenger service in the mountains. These were five batches of
essentially identical locomotives that lasted until the end of steam.
The very last built, AC-12 #4294, rests in the California Railroad
Museum in Sacramento; it is the only surviving Cab Forward.

Class AC-9: Conventional (cab aft) 2-8-8-4 locomotives
ordered for fast freight service. These were coal-burning locomotives
intended for use in the desert where there were no snow tunnels, so the
cab forward configuration was not needed. Designed to the same
technical specifications as the late Cab Forwards, they were converted
to oil burners in the early 1950s and lasted until 1955.

American Steam Locomotive Wheel Arrangements

Here is a summary of the most common American steam locomotive
types. With few exceptions, such as the 2-8-8-2, common wheel
arrangements were named.

Whyte Common Name Notes

0-4-0 Four-coupled Switch engine

0-4-4T Forney four-coupled Industrial use

0-6-0 Six-coupled Switcher

0-6-4T Forney six-coupled Industrial use

0-6-6-0 none First articulated Mallet in USA

0-8-0 Eight-coupled Switcher

0-8-8-0 Angus Low speed helper locomotive

0-10-0 Ten-coupled Heavy switcher

0-10-2 Union Heavy switcher; 5 built for Union RR, Pittsburg

2-4-4-2 none Smallest articulated, used for logging

2-6-0 Mogul Freight

2-6-2 Prairie Dual use

2-6-4T none Suburban passenger service

2-6-6-2 none Freight

2-6-6-4 none Freight

2-6-6-6 Allegheny Freight

2-6-8-0 none Freight, used by GN and Southern

2-8-0 Consolidation Freight

2-8-2 Mikado Freight

2-8-4 Berkshire Freight, first super-power locomotive

2-8-8-0 Bull Moose Freight

2-8-8-2 none Freight

2-8-8-4 Yellowstone Freight

2-8-8-8-2 Triplex Freight (one 2-8-8-8-4 was built)

2-10-0 Decapod Freight

2-10-2 Santa Fe Freight, Santa Fe RR was first user

2-10-4 Texas Freight, Texas & Pacific RR was first user

2-10-10-2 none Freight, used by Santa Fe and Virginian railroads

4-2-2 Bicycle Dual purpose

4-4-0 American Dual purpose, widely used early type

4-4-2 Atlantic Passenger

4-4-4 Jubilee Passenger

4-6-0 Ten-wheeler Passenger

4-6-2 Pacific Passenger

4-4-4-4 Duplex Passenger

4-4-6-2 none Passenger

4-4-6-4 Duplex Freight

4-6-6-2 Cab Forward Passenger

4-6-6-4 Challenger Dual purpose

4-8-0 Twelve-wheeler Freight, also called Mastodon

4-8-2 Mountain Passenger, NYC called theirs Mohawk

4-8-4 Northern Passenger, Northern Pacific was first to use,

            some railroads used other names

4-8-8-2 Cab Forward Dual purpose

4-8-8-4 Big Boy Freight

4-10-0 Mastodon Freight, one built for Central Pacific

4-10-2 Southern Pacific Freight, Southern Pacific was first to use

4-12-2 Union Pacific Freight, Union Pacific was first to use

6-4-4-6 The Big Engine Passenger, one built for PRR

6-8-6 none Dual purpose, one built for PRR

Summary

You now have a basic overview of steam locomotive development, with
a special look at the Southern Pacific’s unique Cab Forwards. For
further information, check the books and other resources below.

Resources

Railroad Books & Videos:

  • If you want more information on steam locomotives, this is probably the first book to read: Guide to North American Steam Locomotives by George H. Drury, Kalmbach Publishing Company (December 1993), ISBN-10: 0890242062
  • American Steam Locomotive
      by Brian Solomon, MBI (May 23, 1998), ISBN-10: 0760303363
  • Cab-Forward: The Story of Southern Pacific Articulated Locomotives by Robert J. Church, Central Valley Railroad Publications; Revised Edition edition (1982), ASIN: B0006EDVP6
  • Perfecting the American Steam Locomotive by J. Parker Lamb, Indiana University Press (June 2003), ISBN-10: 0253342198
  • Model Railroader Cyclopedia: Steam Locomotives by Linn Westcott, Kalmbach Publishing Company (June 1980), ISBN-10: 0890240019

Websites and Online:



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Posted by donna on Jan 30th, 2008 and filed under History, Travel, Uncategorized. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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