NEB&W Guide to Steam Locomotives

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Locos Table of Contents
Rolling Stock Table of Contents


There have been radical changes in the hobby since I wrote this and I have just started going through the rest of this to bring it up to date.

What with the growing interest in steam-era freight cars, hopefully the manufacturers will get the idea that these modelers need steam locos. I know a number of modelers who would like to model, say, 1939, but they realize they can't the steam roster they want, so they go for the 1950's, so they can use diesels to flesh out their roster.


There are some basics about steam that will help you appreciate these. I was born in 1950, just too late to see steam in regular service, but have seen it on fan trips. I try to tell our new members about how awesome a steam engine is in action, but frankly, every time I see one (which isn't often), I realize that I had forgotten just how awesome. At some time I got to visit the B&O museum in Baltimore, and I must admit that I was most impressed by the small engines, ones from the 1830's that were taller than they were long. (One forgets also how tall railroad equipment is. A loco cab is almost up at a second story level.) While these early engines are so dinky by later locos, I think about how these engines must have looked to a group of farmers and everyday people, who had never seen anything bigger than a horse move.

Hero of Greece in the ancient B.C. used steam to do some parlor tricks, but as he lived in a slave society, he and his peers felt it was beneath them to invent anything practical or labor-saving. During the middle ages, as the forests of Europe were over-forested, they had to turn to coal. As they dug the shafts ever deeper, the shafts began to fill with water. In 1698, Thomas Savery patented an engine for pumping out the water. But his engine basically used steam pressure to push against the water directly.

Around 1705, Thomas Newcomen of England invented a heat engine to pump out the water. He came up with the idea of using a piston in a cylinder. This was essentially a vacuum engine. Steam was introduced into a cylinder and piston arrangement, but the steam was just enough above atmospheric pressure to displace the air. Then cold water was sprayed in, causing the steam to condense, creating a partial vacuum and sucking the piston down.

James Watt tinkered with a Newcomen, and saw ways to improve this, which he did over a period of time, starting around 1763. His improvements were so significance that he is given the credit for the whole invention. One major improvement was to separate the cooling out from the main cylinder, creating a condenser, a kind of "anti-boiler". (A boiler converts water to steam, the condenser does the reverse.) This allowed the cylinder to stay hot, and when the piston reached the end of the stroke, a valve opened to allow some of the steam to escape into the condenser. As the steam entered the condenser, it was cooled, sucking more steam into the condenser, and bringing down the pressure of the cylinder/condenser. Watt also gradually raised the pressure of the steam, so that more of the work was done by the expansion of the steam, until finally the proportion done by condensing the steam was so low it wasn't worth having the condenser. It should be pointed out that the Newcomen engine was built in an era when they couldn't make pressure fittings or pressurized boilers, but only as they gained experience with this engine were later improvements in the plumbing possible.

The Watt engine had significantly more power. It also gained in speed, as no longer did the power stroke have to wait for the cylinder to have to heat up and cool down. In 1781, he came up with the idea of a double acting cylinder and by 1788, devised a way to make rotary motion instead of just the idea of up and down motion suitable for a pump. With these improvements, the idea of using the engine to move something began. The first practical idea was to move a boat. Land transportation took a couple of decades later, and had to await developments in the track system, to bring the friction down low enough that these low-powered engines could move itself and a payload.

By the time locos were being built, the condenser was a thing of the past, but I was surprised to find that early steamboats had condensers, and essentially were powered by these vacuum engines.

1900 Watershed

I took a course on the history of technology, where great emphasis was placed on the invention of the whistle and the "cowcatcher". These were trivial inventions. (I was taught that George Whistler - whose wife was immortalized by the painting their son did of his mother - worked on the Long Island RR, where he added a steam-powered trumpet. The name "whistle" was derived from his name. Well, I think he did have something to do with encouraging the use of a whistle, but Ben Franklin wrote about a "penny whistle" he craved when a child, so the name goes back at least a century earlier.)

The development of superheating around 1900 has been credited by those who know as the most significant advance, second only to the invention of the machine in the first place. Superheating was an internal change, but had more visual impact via consequences, so it is worth understanding. Basically, it required the change from slide valves and their square box look to a piston valve with is housed in a cylindrical casing like the piston itself.

Wheel Arrangement

Diesels are classified by their builder and builder model designation. Steam was classified by their wheel arrangement. A Baldwin or Alco USRA 2-8-2 looks pretty much the same at first and second glance. The designation is by wheels, not axles, so the numbers will always be even.

To simplify matters, steam loco wheel arrangements were given nams, generally honoring the first use of that arrangement. But later on, railroads began to pick and choose what name. For instance, the New York Central prided itself on its water level route, thus decided they couldn't have "Mountains". So they called 4-8-2's "Mohawks". When it came to 4-8-4's, it was every railroad for themselves.

But in general, these names are useful for modelers.

Steam engines were divided into three types, based on their intended service: switching, road freight, and passenger. With diesels, the wheels were normally about 40 inches in diameter, across the board. But that's because the wheels were connected to electric "tractive" motors. Steam loco driving wheels were connected to the rodding that powered them from the cylinders. They had to be counterbalanced by extra weight added on the opposite side of the driving wheel from the rod, and even then they tended to pound the rails at speed, actually putting a kink in the rail on each rotation. Loco builders knew how to balance the weight while the engine was at rest, called static balancing. But in motion, the counterbalance was on the face of the wheel and the rods were further out. This makes the arrangement sort of wobble.

As part of a fix, the wheels were made bigger if the engine was designed for speed. A rule of thumb is the maximum speed in miles per hour was roughly equal to the driver diameter in inches. In actuality, a 51-inch drivered switcher would have had a hard time going anywhere close to 51 m.p.h.'s, while an 80-inch Pacific or Hudson could race faster than 80 m.p.h.'s, but this rule is useful.

The boiler, particularly the firebox, is where the power of the engine is developed, and the longer the boiler and bigger the firebox, the more powerful a loco. But for a given boiler length, the builder could put a few big wheels or a lot of tiny ones. For instance, based on the USRA locos, Athearn took their USRA light Mikado 2-8-2 loco and changed the mechanism to make a 4-6-2 Pacific, using the same superstructure.

The lead truck of a steamer, the tiny wheels up front, often called the "pony truck" from the word derived from "small animal", had a purpose. It served to guide the rigid wheelbase of the loco around a curve. A switcher had no pony truck, a freight engine had a two-wheel truck, and a passenger engine had a four-wheel truck. The four wheel truck was more stable and thus functioned better at speed (just like most American freight cars had four wheel trucks for better stability over our rough cheaply built track). Toward the end of steam, they figured out how to make the two-wheel truck more stable, and the famous NKP Berkshires (2-8-4's) were high-speed freight engines.

But the second wheel set of a four wheel lead truck was behind the cylinders, taking up space from the length available for the driving wheels. So a Pacific and Mikado fit under the same boiler. (Well, sort of, the same wheel arrangement fits in terms of length. Putting fewer but bigger wheels, such as a Pacific mechanism under a Mikado boiler raises the overall superstructure. On the prototype, they would lower the domes to fit within clearances. The model makers just produce a too-high boiler.)

As mentioned above, the size of the firebox is the true indication of the power of the engine. At first the firebox was rested above the drivers (spreading it out) or sometimes it was placed between the drivers, making the firebox deep but narrow. (If you look at most ten-wheelers, the last pair of drivers is spaced further from the middle set to make room for the firebox.) By WWI, designers were placing the firebox behind the drivers, which required a small wheel set to support the weight, the trailing truck. By the end of the '20's, they had taken it further, by making the firebox even longer and requiring a four-wheel trailing truck. (Loco models sometimes change a four wheel trailing truck for a two wheel one, or vice versa, to make more wheel arrangements available. For instance, Rivarossi took their streamlined 4-6-4 and used it to model a LV streamlined Pacific partly by changing the lead truck to a two wheel one. And Mantua took their Pacific and Mikado models and offered them as Hudsons and Berkshires, respectively, by squeezing in a four wheel trailing truck. Knowledgeable steam fans would recognize this deception, as the length of the firebox doesn't match the new trailing truck.

Loco Geometry

On of the early advances that made steam locomotion possible was to use the steam exhaust to increase the draft on the fire. The steam was directed up the stack, and in doing so sucked air from the fire. What you see as "smoke" is mostly steam, which is why the exhaust is so much more intense on a cold winter day. Also, this is why a steam locomotive goes "choo choo" so much more than a steam boat or other steam powered device does.

This means that the middle of the cylinder has to be in line with the stack.

Another rigid geometry is that the cylinders have to be level and have to be in line with the center of the drivers. If the cylinders were canted (as they were on a few early engines), the connecting rod would be delivering a hammering blow down to the rails with each revolution. If the cylinders weren't in line, the power stroke would be uneven at the top and bottom of the stroke.


A wide firebox, used to burn the slower-burning anthracite coal, was felt to require the cab placed over the boiler in front of the firebox. I believe the design was later banned (in the 1930's) by the ICC and late-steam locos had massive fireboxes AND boilers, yet the engineer was able to see from a rear cab. However, for more info, see this section.

Harriman Designs

C. 1900, the Harriman lines (including the SP, UP, Alton, and IC) shared common motive power and rolling stock standard designs. The locos were typical of the times so these models are fairly useful even if you don't model any Harriman line. There is one problem in that the Harriman cab had a distinctive scalloped cutout in the overhang, which is tricky to modify to represent locos of other roads.

USRA Designs

During the first World War, the government took over the railroads, under the auspices of the United States Railway Administration. While they appear to have made a mess of the operations, they had design committees composed to leading railroad mechanical engineers who put aside most of their proprietary pettiness to come up with standard designs. During the War, locomotives (and rolling stock) built to these specific plans were distributed among different railroads. The designs were so good, they continued to influence the industry for years and years, so you will see the terms USRA clones or copies. As time passed, while the basics of the designs were followed, the cosmetics and details were altered to keep up with changing trends. Therefore, from a modeling standpoint, these later engines sometimes bear little resemblance to the original designs.


The idea of streamlining goes back to the 1890's. There was the so-called "Adams Wind-Splitter". But how much energy it saved wasn't known.

By the late '20's, airplanes had increased in speed enough to where aeronautical engineers came up with the idea of a wind tunnel where they could test the amount of drag of different designs. By using a smoke machine, they could see how the air streamed around different shapes. Some smooth shapes gave free-flowing lines, while other shapes, particularly projections, created turbulence. The term "streamlining" came from this means of testing.

We are so used to the concept it is hard to realize how radical this was. Airplanes represented the epitome of technology at the time and their distinctive smooth shape spilled over to everything, EVERYTHING. Automobiles were given a streamline shape, locomotives, even buildings, which were are stationary.

The Burlington Zephyr made national headlines. Not only was it the first speedy diesel, the entire train was shaped like an airplane without wings. Locomotive makers scrambled to follow suit. Some extra sheet metal was used to make a shroud to cover all the piping, domes, projections.

The early streamlined locos basically covered everything. Within a few years, they realized the cowling over the running gear hindered maintenance, so later designs left this exposed, while some of the earlier locos had the sheet metal removed. Then during the War, more sheet metal fell prey to the scrap drives. Few streamlined steam locos were built after WWII. (Few steamers, period, were built, but the idea of them being thoroughly modern had passed to diesels.)