Southern Unionist
First Sergeant
- Joined
- Apr 27, 2017
- Location
- NC
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.
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.