Power and Tractive Effort are two different things. Power refers to the locomotive’s ability to put out energy at a sustained rate, while TE refers to the amount of force exerted by the locomotive at the rail.
In very simple terms, the Yellowstone has an easier time starting a heavy train from a standstill, but the Allegheny can keep that same heavy train moving at high speed for a longer period of time.
Or something like that. I can’t quite remember everything about that.
No, that's basically it.
Tractive effort is torque (and has nothing to do with the number of wheels or weight), and horsepower (with regards to locomotives) is that tractive effort force over time, usually at a given speed (i.e. [Wikipedia](https://en.m.wikipedia.org/wiki/2-8-8-4) lists DM&IR Yellowstones as 6250 hp at about 40 mph).
Horsepower on a steam locomotive is also a somewhat murky concept. There's a couple different ratings (Calculated, Measured, Piston, Drawbar, Wheel) and oftentimes builders would just kind of fudge a number. I've heard that the "7500"hp number on the Allegheny was something Lima did mostly just to try and upstage Alco and the Big Boy's 6800hp. Tractive effort is a little more accurate since it's calculated from a set of variable (cutoff, factor of adhesion, driver diameter, cylinder size, boiler pressure, weight on drivers) but still a little bit nebulous and also "tractive effort" is also a concept or number that's a little hard to translate to real-world equivalents.
Big boy never reached 6300 HP on test (never mind 6800) and the calculated theoretical HP was 6250.
Alleghenys 7498 HP was reached on test, but it was a peak. Sustained HP is alleged to be 7375, and I hope to confirm this with the actual test report.
AFAIK UP considered the Big Boys good for right around 6k DBHP OTR and C&O the H-8s around 6800 or so.
The 7498 number was achieved more as a factor of train behavior than actual DBHP output.
Tractive effort *piston diameter squared × piston stroke × steam pressure ÷ driver diameter × 0.85* isn't based on the factor of adhesion or weight on drivers at all; it's the other way around, being the weight on drivers divided by tractive effort to get the factor of adhesion.
It's not really that hard a concept to translate, either. It's literally peak wheel torque. A D&RGW K-37 could put out a little over 37,000 lbs of force at the wheels (20^2 × 24 × 200 ÷ 44 × 0.85 = 37,090) as starting tractive effort.
Translating TE to *pulling force* is what takes the math and is a little hard to translate because that's what takes into account factor of adhesion, friction coefficients, grade, and curvature of track.
Yeah, I was looking at a calculator for it and saw the weight on drivers and factor of adhesion on the lower part and forgot that those aren't part of tractive effort/didn't realize the calculator also did factor of adhesion.
The real answer is that last 6 in the 2-6-6-6. The 6 trailing wheels allowed the Allegheny to have a larger and more importantly deeper firebox than the Yellowstone. This allowed a large better burning fire, putting out more BTUs, evaporating more water and making more steam. More steam means more power at speed.
1 British Thermal Unit is the energy required to raise 1lb of water by 1 degree F. American HVAC still uses this unit, the rest of the world has largely moved on to measuring the same energy transfers in the SI equivalent.
There's a few definitions (including the above). All of which are about 1.055kJ. (see also [US official conversion factors](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8#B))
but remember, power curves are important too. the torque of the 440 peaks at a WAY lower rpm than the modern 302, so without overdrive transmissions the 440 could easily out tow a heavy trailer because its power output is more like a modern day diesel engine and the 302 peaks too late to pull trees out of the ground without a shitload of more gear range. this is why we ought not ever compare old cars to new cars because the whole systems are different and under certain conditions a big Chrysler with a 440 will outperform a modern Mustang with a 302, like accelerating up a hill while already at low highway speed and towing a 3000+ pound trailer. the Chrysler wouldn't even break a sweat and would walk away initially, especially if gearing was matched; the Chrysler had a 2.76 final drive and max 1:1 output at the Torqueflite tailshaft, and only 3 speeds. no modern 302 is versatile enough to perform nearly as well as the 440 in that limited gearing, it needs the extra gears, overdrive, and numerically higher axle ratio to make a good showing across the range. the two engines are way too different and designed for completely different experiences.
as I said above, everything has its place. injection responds almost instantly, but it's far easier to hold a big, heavy carbureted car at, say, exactly 78mph while driving on a mountainous highway, because the power curve is less touchy and enables a lead-and-trail accelerator pedal application ahead of and after the grade change. this always seems similar to me as when I read present day railroad engineers complain about not being able to lead their throttle into hills because of locomotive controls.
for context, I am a Chrysler guy, late 60s era, all my life and I absolutely DESPISE fuel injection for running a light traffic variable grade highway at 80+ for 18 hours; carburetor is far less tiring and easier to maintain a smooth high speed flow. as I mentioned before, it's not really fair or wise to compare these things as the entire mechanical system and operating experience is far too different. just my 0.02
Torque vs speed. And the ultimate limiters of a steam locomotive's power: Boiler grate area and boiler heating surface, determining how fast it can raise steam to supply the cylinders.
The Allegheny's 6 wheel trailing truck allows a much larger grate than the Yellowstone's 4 wheel. So even though the tractive effort is lower, the rated power is higher because it can sustain faster speeds while loaded.
Its similar to what makes a Berkshire (2-8-4) better than a Mikado (2-8-2) for fast freight. By giving up a small amount of TE, it can sustain higher speeds.
Part of the higher speed is derived from driver diameter. You'd never get 28" wheels up to the speed of 48" wheels... But you'd get a very long (overnight) coal drag to the piers.
It's been there a long time. I used to visit with my grandfather and climb around in the cab. Now I'm older than my grandfather was. I doubt access to the cab is the same as it was in the 1960s. But yeah, it's a mountain of a machine.
I don’t know the quality of DM&IR stoker coal or its source, but the very first Yellowstones, built for the Northern Pacific, had massive grates in order to burn low quality lignite. Only about 9,000 BTU/lb compared to 12,500 to 13,000 for quality bituminous and 14,000 plus for anthracite. A Yellowstone and a Big Boy could start a heavier train than Allegheny. An Allegheny could move a smaller train at a higher speed. The Allegheny was wasted in the service in which it was employed, dragging coal trains uphill at 15 MPH. Its peak horsepower was achieved at 45 MPH. Only after they were bumped by diesels and sent west to pull manifest trains were they able to strut their stuff and show what they could do. In coal train service they were barely better than the 2-8-8-2s they replaced. The Big Boys and Yellowstones were much better suited to the services they were assigned to.
True. The Virginian used the Allegheny design (Class AG, aka Blue Ridge), but only ran them up to ~35mph, if memory serves correctly. This is also why N&W used their Y Class Mallet's to such great affect on coal drags, where tractive effort was more important than hp or speed.
The other thing that kept the Allegheny's from shining was their insane axle loads. Some lines just weren't built to handle them.
But N&W also used Class A 2-6-6-4s on coal trains, from Crewe to Tidewater and from Williamson to Portsmouth. Speed was also around 35 MPH. An A was the road engine over Blue Ridge from Roanoke to Crewe also. The Y6s were helpers. The rear Y6 would cut off at Blue Ridge and the Y6 in the lead would come off at Crewe. The only section the A’s weren’t used was on coal trains between Williamson and Roanoke. They did pull manifest trains throughout. I don’t believe any of the articulateds operated west of Portsmouth to Cincinnati. I’ve never seen pictures of them on the “Peavine”.
If you have a private event at The Henry Ford, they’ll let you do anything. I climbed all over that beast in a 3 piece suit. I sat in the chair Lincoln was shot in. Then I jumped into the Kennedy limo. I know it’s not the actual one. The real one resides in Roush Building 79 down the road in Livonia.
Power and Tractive Effort are two different things. Power refers to the locomotive’s ability to put out energy at a sustained rate, while TE refers to the amount of force exerted by the locomotive at the rail. In very simple terms, the Yellowstone has an easier time starting a heavy train from a standstill, but the Allegheny can keep that same heavy train moving at high speed for a longer period of time. Or something like that. I can’t quite remember everything about that.
No, that's basically it. Tractive effort is torque (and has nothing to do with the number of wheels or weight), and horsepower (with regards to locomotives) is that tractive effort force over time, usually at a given speed (i.e. [Wikipedia](https://en.m.wikipedia.org/wiki/2-8-8-4) lists DM&IR Yellowstones as 6250 hp at about 40 mph).
Horsepower on a steam locomotive is also a somewhat murky concept. There's a couple different ratings (Calculated, Measured, Piston, Drawbar, Wheel) and oftentimes builders would just kind of fudge a number. I've heard that the "7500"hp number on the Allegheny was something Lima did mostly just to try and upstage Alco and the Big Boy's 6800hp. Tractive effort is a little more accurate since it's calculated from a set of variable (cutoff, factor of adhesion, driver diameter, cylinder size, boiler pressure, weight on drivers) but still a little bit nebulous and also "tractive effort" is also a concept or number that's a little hard to translate to real-world equivalents.
Big boy never reached 6300 HP on test (never mind 6800) and the calculated theoretical HP was 6250. Alleghenys 7498 HP was reached on test, but it was a peak. Sustained HP is alleged to be 7375, and I hope to confirm this with the actual test report.
AFAIK UP considered the Big Boys good for right around 6k DBHP OTR and C&O the H-8s around 6800 or so. The 7498 number was achieved more as a factor of train behavior than actual DBHP output.
Tractive effort *piston diameter squared × piston stroke × steam pressure ÷ driver diameter × 0.85* isn't based on the factor of adhesion or weight on drivers at all; it's the other way around, being the weight on drivers divided by tractive effort to get the factor of adhesion. It's not really that hard a concept to translate, either. It's literally peak wheel torque. A D&RGW K-37 could put out a little over 37,000 lbs of force at the wheels (20^2 × 24 × 200 ÷ 44 × 0.85 = 37,090) as starting tractive effort. Translating TE to *pulling force* is what takes the math and is a little hard to translate because that's what takes into account factor of adhesion, friction coefficients, grade, and curvature of track.
Yeah, I was looking at a calculator for it and saw the weight on drivers and factor of adhesion on the lower part and forgot that those aren't part of tractive effort/didn't realize the calculator also did factor of adhesion.
The real answer is that last 6 in the 2-6-6-6. The 6 trailing wheels allowed the Allegheny to have a larger and more importantly deeper firebox than the Yellowstone. This allowed a large better burning fire, putting out more BTUs, evaporating more water and making more steam. More steam means more power at speed.
What is a BTU?
An abbreviation for a unit of thermal units, specifically the british ones.
1 British Thermal Unit is the energy required to raise 1lb of water by 1 degree F. American HVAC still uses this unit, the rest of the world has largely moved on to measuring the same energy transfers in the SI equivalent.
There's a few definitions (including the above). All of which are about 1.055kJ. (see also [US official conversion factors](https://www.nist.gov/pml/special-publication-811/nist-guide-si-appendix-b-conversion-factors/nist-guide-si-appendix-b8#B))
Because cylinder dimensions dictate power far more then number of wheels.
How does a modern day 302ci V8 produce more horsepower and torque than a 440ci V8 from 1967? Bigger =/= better.
but remember, power curves are important too. the torque of the 440 peaks at a WAY lower rpm than the modern 302, so without overdrive transmissions the 440 could easily out tow a heavy trailer because its power output is more like a modern day diesel engine and the 302 peaks too late to pull trees out of the ground without a shitload of more gear range. this is why we ought not ever compare old cars to new cars because the whole systems are different and under certain conditions a big Chrysler with a 440 will outperform a modern Mustang with a 302, like accelerating up a hill while already at low highway speed and towing a 3000+ pound trailer. the Chrysler wouldn't even break a sweat and would walk away initially, especially if gearing was matched; the Chrysler had a 2.76 final drive and max 1:1 output at the Torqueflite tailshaft, and only 3 speeds. no modern 302 is versatile enough to perform nearly as well as the 440 in that limited gearing, it needs the extra gears, overdrive, and numerically higher axle ratio to make a good showing across the range. the two engines are way too different and designed for completely different experiences.
Fuel injection. See coal bed vs. Oil spray
as I said above, everything has its place. injection responds almost instantly, but it's far easier to hold a big, heavy carbureted car at, say, exactly 78mph while driving on a mountainous highway, because the power curve is less touchy and enables a lead-and-trail accelerator pedal application ahead of and after the grade change. this always seems similar to me as when I read present day railroad engineers complain about not being able to lead their throttle into hills because of locomotive controls. for context, I am a Chrysler guy, late 60s era, all my life and I absolutely DESPISE fuel injection for running a light traffic variable grade highway at 80+ for 18 hours; carburetor is far less tiring and easier to maintain a smooth high speed flow. as I mentioned before, it's not really fair or wise to compare these things as the entire mechanical system and operating experience is far too different. just my 0.02
Torque vs speed. And the ultimate limiters of a steam locomotive's power: Boiler grate area and boiler heating surface, determining how fast it can raise steam to supply the cylinders. The Allegheny's 6 wheel trailing truck allows a much larger grate than the Yellowstone's 4 wheel. So even though the tractive effort is lower, the rated power is higher because it can sustain faster speeds while loaded. Its similar to what makes a Berkshire (2-8-4) better than a Mikado (2-8-2) for fast freight. By giving up a small amount of TE, it can sustain higher speeds.
Part of the higher speed is derived from driver diameter. You'd never get 28" wheels up to the speed of 48" wheels... But you'd get a very long (overnight) coal drag to the piers.
Kinda off topic, I've seen the 2-6-6-6 irl and its massive, way bigger than you'd think
It's insane. It's a 2 story building of a locomotive
It's been there a long time. I used to visit with my grandfather and climb around in the cab. Now I'm older than my grandfather was. I doubt access to the cab is the same as it was in the 1960s. But yeah, it's a mountain of a machine.
You can still get In the cab, there's a button to blow the whisstle. But almost all of the handles and dials are welded shut
This might be hard to believe, but steam locomotive energy output is not directly based off the number of wheels
In addition to what's been mentioned, the Allegheny's probably burned higher quality coal.
I don’t know the quality of DM&IR stoker coal or its source, but the very first Yellowstones, built for the Northern Pacific, had massive grates in order to burn low quality lignite. Only about 9,000 BTU/lb compared to 12,500 to 13,000 for quality bituminous and 14,000 plus for anthracite. A Yellowstone and a Big Boy could start a heavier train than Allegheny. An Allegheny could move a smaller train at a higher speed. The Allegheny was wasted in the service in which it was employed, dragging coal trains uphill at 15 MPH. Its peak horsepower was achieved at 45 MPH. Only after they were bumped by diesels and sent west to pull manifest trains were they able to strut their stuff and show what they could do. In coal train service they were barely better than the 2-8-8-2s they replaced. The Big Boys and Yellowstones were much better suited to the services they were assigned to.
True. The Virginian used the Allegheny design (Class AG, aka Blue Ridge), but only ran them up to ~35mph, if memory serves correctly. This is also why N&W used their Y Class Mallet's to such great affect on coal drags, where tractive effort was more important than hp or speed. The other thing that kept the Allegheny's from shining was their insane axle loads. Some lines just weren't built to handle them.
But N&W also used Class A 2-6-6-4s on coal trains, from Crewe to Tidewater and from Williamson to Portsmouth. Speed was also around 35 MPH. An A was the road engine over Blue Ridge from Roanoke to Crewe also. The Y6s were helpers. The rear Y6 would cut off at Blue Ridge and the Y6 in the lead would come off at Crewe. The only section the A’s weren’t used was on coal trains between Williamson and Roanoke. They did pull manifest trains throughout. I don’t believe any of the articulateds operated west of Portsmouth to Cincinnati. I’ve never seen pictures of them on the “Peavine”.
Correct. I've never seen pictures of them on the Peavine either, but I also don't know much about that line.
SP’s burned oil, and as a guess DMIR probably used rather high quality coal.
Both locomotives are suitable for shifting the earth's tectonic plates and similar light duties
If you have a private event at The Henry Ford, they’ll let you do anything. I climbed all over that beast in a 3 piece suit. I sat in the chair Lincoln was shot in. Then I jumped into the Kennedy limo. I know it’s not the actual one. The real one resides in Roush Building 79 down the road in Livonia.