Steam Locomotive Wheel Configurations

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Significant steam locomotive wheel configurations (excluding specialty locomotives for steeper grades):

0-4-0: Earliest functional steam locomotives, such as the B&O “Grasshopper” and the “Best Friend of Charleston” in SC. These engines had four drive wheels (two powered axles), no guide wheels of any kind, and an upright boiler on an open platform, resembling a moonshine still on rails.

Though having all the locomotive’s weight on powered axles helped with traction, these engines tended to wobble badly from side to side on straight track, especially if the wheelbase was short. The space between the rails had to be wider than the distance between the wheel flanges, allowing opportunities for unwanted rotational movement, also known as “hunting”. Also, the lead axle was constantly trying to climb over the outside rail on curves, especially when sand was being used to help with traction (in later models), as inertia made the engine want to go straight. Thus, the top safe speed of these engines was severely limited.

The 0-4-0 wheel configuration survived in the long term only in yard switching engines, which never went fast, never traveled on curvy track, and needed as much traction as possible to get long strings of cars moving.


0-6-0: Common yard switcher, later in the steam era.


0-8-0: Rare, super heavy duty yard switcher.


2-4-2 / 4-2-0 / 2-2-2 / 2-4-0: Early experiments with guide wheels, in order to achieve higher safe speeds. Sets of four front guide wheels were attached via swivel mount, while single pairs were semi-rigid. Powered axles were always rigidly attached to the frame, while trailing guide wheels were mounted on a trailing arm style pivoting triangular frame, with a slide plate under the cab. Once these suspension standards were established, they lasted throughout the steam era.


4-4-0: The closest thing America ever had to a standard locomotive. Before, during, and after the Civil War, it was the locomotive of choice for nearly every assignment. The configuration looked good on paper, and it proved itself in the real world, handling sharp curves, curve transitions, and poor track conditions quite well. Railroads were so happy with them that alternatives to the design were not seriously considered for decades. The only obvious weakness was a speed restriction on backing up.

The design is somewhat of an optical illusion. It looks nearly balanced, but most of the heavy stuff including the firebox and most of the boiler is over the drive wheels. The smokebox up front (under the stack) is hollow space. The guide wheels only needed enough weight on them to properly steer the frame and take most of the stress off the drive wheel flanges. Not only did the locomotive perform better than the older designs, but rail wear was significantly reduced.


2-6-0: The first significant step away from standardization, after the war. In hilly terrain, longer freight trains had lower speeds. Two guide wheels up front seemed to be enough in that situation, and six drive wheels provided better traction. Passenger trains stuck with the 4-4-0 design, in most situations.


2-8-0: The logical next step beyond the 2-6-0. It quickly became America’s standard low speed branch line freight engine, a niche that it filled until the diesel era. Thousands were built. They also pulled mixed freight and passenger trains on low speed branch lines. Maxing out at around 2000 HP, it was a perfect match for the assignment, in every way, including economics. Today, it is the most common surviving steam engine configuration.


2-10-0: Rare, heavier, mountain version of the 2-8-0.


2-8-2: Larger, heavier version of the 2-8-0, used on mainline freight. Trailing guide wheels improved curve transition stability at higher speeds, helped support the weight of longer and heavier fireboxes, and allowed higher speeds in reverse. Built in large numbers.


2-8-4: Relatively rare variant of the 2-8-2, designed for long mountain grades where a larger and heavier firebox is needed, for a larger fire to maintain steam pressure.

4-6-0: This transition locomotive for passenger and mail trains came in between the 4-4-0 and the 4-6-2 on many railroads.

4-6-2: The configuration that finally replaced the 4-4-0 on passenger trains, once and for all. Once the designers settled on this configuration for passenger train engines, they shifted their focus to making it bigger, faster, and more powerful. It holds all the important speed records for the steam era. In flat regions, the newest and most advanced designs could pull trains in excess of 100 miles per hour.


4-8-4: Larger and heavier, higher powered version of the 4-6-2, for larger and heavier passenger trains on steeper grades, almost as fast as the 4-6-2. Some were used for high speed priority freight.


2-6-6-2 / 2-8-8-2: These were among the largest, heaviest, and most powerful steam locomotives ever built, anywhere in the world. These 4-cylinder designs had two sets of drive wheels, with the rear set of 6 or 8 wheels rigidly attached to the main frame, while the forward 6 or 8 rode on a subframe that featured a rear hinge and multiple slide plates ahead of the hinge that were designed to bear tremendous weight loads. The front subframe assembly had the ability to swing several feet out from under the nose of the boiler, creating a bizarre appearance on sharp curves. That assembly functioned as guide wheels for the rear half of the engine, enabling good stability at good speeds.


2-6-6-4: Higher speed freight version of the above designs, for priority trains such as fruits and vegetables moving from the Southeast to the Midwest. The world’s highest horsepower surviving steam engine is a 2-6-6-4, the Norfolk and Western #1218.


This is as far as we got with transportation engineering technology before diesel / electric locomotives became dominant.
 

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That photo of the 2-8-8-4 is impressive, the largest engine that I have seen was at the York railway museum, it was a Chinese 4-8-4 KF and it dwarfed everything around it, it was 15 feet tall and 93 feet long but it’s just a baby compared to that 2-8-8-4. I’m betting that seeing something that big in full steam and motion is breathtaking.
 
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just a baby compared to that 2-8-8-4. I’m betting that seeing something that big in full steam and motion is breathtaking.
Norfolk & Western's 2-6-6-4 design was just slightly higher in horsepower, designed to maintain higher average speeds in the mountains with high priority mixed freight, while the heavier 2-8-8-4's were assigned only to low speed coal drags. N&W's #1218 was for many years the highest horsepower operating steam engine left in the US, and I saw it pull several railfan excursions. It was incredible to watch it "bend" around curves. I rode behind it once, and its only shortcoming for pulling long passenger trains (18+ cars) was when the front and rear drive wheels happened to get in sync on a hard pull; you could feel slight power surges. The engineer would fix this by turning off the front sand for a moment and letting the front drive wheels slip just enough to get them out of sync with the others.

That engine is stored in Roanoke, where the N&W 611 spends every winter, and could run again if sufficient funding is found someday. Currently, the 611 (4-8-4) is our most powerful steam engine that still runs on a regular basis. Its limitations on long excursions are getting the coaches started on a grade with only four powered axles, and getting around sharp mountain curves with a ridged four axle wheelbase. The 1218 suffers from neither problem.

Other notable articulated steam engines stored in the East include Union Pacific's "Big Boy" at Steamtown (Scranton) and a C&O "pump face" at the B&O Museum in Baltimore. The only remaining N&W 2-8-8-4 is normally on display in Saint Louis. Steamtown's big engine was received in operating condition.
 

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Very impressive and I was surprised to read that they were coal fired rather than wood. I thought steam locos in the U.S were for the most part wood fired. Intresting point about potential problems with track cohesion with lightened tenders. I suppose that the gradients that these engines were expected to climb negated any traction issues but the issue of mainting coal and water on these beasts must have been a logistical nightmare. I had wondered why the railroads hadn’t just stuck with the tried and tested method of using banking engines but it makes sense when one considers the vast distance these engines were expected to travel. A quick look at other railroads in the rest of the world shows that the preferred method for overcoming steep inclines was the use of a second engine but for the most part the second engine (banking loco) was only a small loco intended for giving a train a quick shove up the hill.
After all of my waffle, what I’m really saying is that as impressive as these massive engines are they just don’t strike me as being practical or cost effective. I’m also guessing that maintaining these locos would have taken them out of service for weeks at a time. Now what I’d really like to see are pictures of these engines on shed, I can only imagine the size of the lifting gear used just to replace a wheel with a flat spot.
 

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I was surprised to read that they were coal fired rather than wood.
Before the ACW, a few roads began the switch to coal - mainly because they had such easy access to tremendous supplies of it and easily obtainable wood was getting scarce. After the ACW, the change came fast and furious. The 2-10-10-2s were built in 1918 and lasted into the 1940s. The 2-8-8-8-4 in 1916 and only lasted till 1920. The 2-8-8-8-2s were built between 1914 and 1916. Not sure how long they lasted.

...they just don’t strike me as being practical or cost effective.
They didn't end up being that. That is why they ended up being basically one-off experiments. Really cool looking but not economically viable.

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Before the ACW, a few roads began the switch to coal - mainly because they had such easy access to tremendous supplies of it and easily obtainable wood was getting scarce. After the ACW, the change came fast and furious. The 2-10-10-2s were built in 1918 and lasted into the 1940s. The 2-8-8-8-4 in 1916 and only lasted till 1920. The 2-8-8-8-2s were built between 1914 and 1916. Not sure how long they lasted.



They didn't end up being that. That is why they ended up being basically one-off experiments. Really cool looking but not economically viable.

Cheers,
USS ALASKA
I’m going to test your knowledge here, how much weight could those engines pull on a straight and level track? And before someone asks, I haven’t got the answer.
 

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