Steam & Excursion > is boiler pressure for a steam loco analogous to hp w/ diesel?

At least to some extent . . .
My question is prompted by a recent thread about NYC Mikado with 200 lb. boiler pressure
So, lots of driven axles for tractive effort and lower BP is adequate to move tonnage (?)
Then, for any wheel arrangement and driver diameter, more BP delivers more track speed (?)
Many thanks for information!
 

eljay Wrote:
-------------------------------------------------------
> At least to some extent . . .
> My question is prompted by a recent thread about
> NYC Mikado with 200 lb. boiler pressure
> So, lots of driven axles for tractive effort and
> lower BP is adequate to move tonnage (?)
> Then, for any wheel arrangement and driver
> diameter, more BP delivers more track speed (?)
> Many thanks for information!

Since my long time buddy Wes Camp is gone, I'll point out a few differences of steam locomotives vs. diesel electric locomotives:

1) Steam locomotives are essentially constant torque (tractive effort), variable horse power machines. That means, if the steam locomotive can just start the train, it is capable of accelerating that train to very great speeds. The steam locomotive actually INCREASES horse power as it accelerates, up to the balance speed based on the drive wheel diameter.

2) The diesel electric locomotive is just the opposite, i.e. it is a constant horse power, variable torque machine. The slower the diesel electric goes the more it can pull. Thus, a diesel electric locomotive is capable staring a much heavier train that it can NOT accelerate to speed.

In my opinion, you question can easily be related to simply comparing a steam locomotive to a diesel electric locomotive, since there is a LOT more involved than just "boiler pressure".

So,.what it s definition of "tractive effort" as that is a major qualifier (but not the only one) in how steam locos are rated. Is that not a good qualifier for diesel locos based on what you just wrote Jack?

Victor Baird

Tractive effort in a steamer was always rated at about 25% of the weight on the drivers. Tractive effort on a
diesel can be whatever the wheelslip detection will allow and you hear nominal numbers from the OEM which
may or may not reflect real life. The common definition is simply drawbar pull and that's pretty uniform in a
diesel as the effecting factors are electronically controlled (assuming dry rail). A steamer was only as good as
the engineer running it. Catching the slip prior to it happening was a real art.  The point on boiler pressure
is that it will give you the torque available at a fixed pressure ie TN/5252=HP. As Jack mentions a steamer
provides uniform torque (at a fixed pressure) ie affecting pressure on a fixed area of each cylinder. As the
formula states, as speed increases so does horsepower as because of the pressure times area thing the T
number remains constant. 

Out 



Edited 1 time(s). Last edit at 11/03/25 20:41 by callum_out.

Tractive effort is a calculation based on boiler pressure, piston diameter and stroke, and driver diameter.  See the following site for more details, be sure you click on the "About Tractive Effort" and "About Factor of Adhesion" buttons on the top to see the actual formulas.  

https://www.steamlocomotive.com/misc/tractiveEffort.php

I also remember an article in Trains a decade or two back titled something along the lines of "Big Boy, Big Success or Big Mistake?" that talked at some length about horsepower curves between steam and diesel.  I remember it as being an interesting article.  

Jeff Moore
Elko, NV

Stroke only relates to driver diameter in that the stroke has be to long enough to handle the required radius on
the driver wheels. That radius does affect tractive effort though stroke is secondary. The radius is essentially
the lever arm that turns the wheels, more lever, longer stroke. All of which becomes moot when the boiler is
no longer able to keep up with the steam requirements ie the ATSF 2-10-10-2 disaster.

Out 

Had an interesting experience back in the 80's with an NS excursion between Erie, PA and Bellevue, OH.   Both ways about 18 cars, NS main line, and before the speed restrictions were put in place.
Outbound from Erie to Bellevue 611 handled the train in stride, track speed 70mph, through Cleveland, along the lake, and arrived at Bellevue on time.   I'd ridden behind 611 before and had been astounded at the accelleration at speed, you could feel it in the seat.   And 611 was rated, austensbly, at something like 5400hp max.   Relatively flat and straight running.   Moved rapidly, and you never even heard sharp exhaust, once it got rolling, get out of the way.

There was a train separation and broken air hose on the diamond at Bellevue, we sat there a long time waiting to depart, over an hour.  Then the fun began.  

611 kept going and the return trip was being handled by two squeaky-clean, factory fresh C30-7's from GE.   OK, so 3000 hp * 2 6000hp.   I was in the open-window coach directly behind the two units with the windows open, nice summer night.  Away we went back east.

You could actually hear the transition and relays firing up front, they would come up to speed and hit full parallel about 45, you'd hear the power straining, and RPM's drop down, relays fire, and slow back down again in transition mode.   Over and over.   We managed to hold a fairly constant 40-45, but those two units NEVER got it up to track speed.   And we got later....and later... and later.   Got back to Erie about three hours late.  Not that the crew didn't try, those two GE's just couldn't deliver the same consist at the same speed.

Still is the best demonstration of steam vs. diesel performance on an indentical route identical train, same day, I've ever seen and explained it perfectly.   Steam horsepower at speed is incredible, diesel horsepower at startup is incredible, but you have to pile more and more power to get speed.  Steam needs more power to start, which is why helpers were often at yards to start a train against an outbound grade, then drop off.     Get steam down on it's knees slow, may have a stall.   Diesel can hold almost anything under about 20mph.   The other demo is looking at the hp/ton ratios of trains expected to hold 90mph like the Super C.   Took tons of horsepower and high gearing, wasn't unusual to have 3 SD45's on 18 - 20 pigs in the early 70's at 3600hp each to do it.

To answer the guy's question, we have to start with a question: do you know what "horsepower" means? Tell us, so we know where we need to start the explanation.

Maybe I shouldn’t attempt this in the morning before fueling up on my ration of coffee, but I’ll give it a try, and try to focus on the OP’s question as related to boiler pressure and speed as related to a steam locomotive.
 
As HotWater stated very succinctly, a steam locomotive can pull a train it cannot start, or barely start a train then accelerate it, while a diesel can start a train it then can no longer accelerate.
 
A diesel locomotive is a machine powered by an engine. A steam locomotive is the engine itself.
 
The diesel engine is not connected mechanically to the wheels, therefore the engine can develop full horsepower regardless of how fast the wheels are turning, or not turning.  Full horsepower can be developed at zero mph if the wheels do not slip. Horsepower is derived from engine RPM (pistons moving faster back and forth within the cylinder).
 
A steam engine develops horsepower on the same principle, the faster the pistons move, the more horsepower it develops. But since the pistons are connected to the wheels, it needs to increase track speed in order to increase horsepower.
 
Now, that said, lets divert to the question asked about tractive effort before going back to steam pressure and speed.
 
Tractive effort is the grunt power or force measured in pounds (for our use here) that it takes to make a static mass or stationary object like a train move; the “pull” power. It can mathematically be defined as  
 
TE = .85 x P x d squared x s x N
_______________________________________
 
                    DD
 
.85 is an accepted efficiency factor
 
P = Boiler Pressure
 
d = piston diameter or cylinder bore
 
s = stroke of the piston
 
N = number of cylinders
 
DD = drive wheel diameter
 
Now we understand tractive effort and how to determine it, what about steam pressure?
 
In this case, within the cylinder the higher the pressure the more tractive effort or torque that can be generated. Generally speaking steam locomotives have ratio of tractive effort to weight on drivers of 25% or 4:1. So a Factor of Adhesion of 4.0 is the generally accepted target. Less than that and the locomotive can be slippery. The easy solution then is to reduce the boiler pressure. If the Factor of Adhesion is over 4.0, say 5.0 then the locomotive is too heavy for the theoretical TE power it can generate and is wasting energy in having to move or start its excess weight.
 
Also to be understood is that in this case, the steam pressure is only related to the work performed in the cylinder as it relates to the pull power. The more pressure the more saturated the steam and the more expansive (powerful) a given amount or volume of steam will be as it vents out of the boiler to atmosphere.
 
So the more pressure, the more energy that is in a given volumne of steam. However, this doesn’t translate to speed, but translates to efficiency, one of the reasons for superheating steam and related to the D&H’s L.F. Loree designed high pressure experimental steam power of the 1930’s.  The higher pressure steam while gaining an efficiency also ran into trouble with packing (seals) and breakdown of lubricating oils among other things. So there became a diminishing law of returns by doing it.
 
Speed of a steam locomotive is determined, as previously mentioned, by the RPM of the pistons which are directly mechanically coupled to the driving wheels, so the faster the pistons move, the faster the wheels turn and the faster the locomotive goes. So what limits that if it is not boiler pressure? Well, it is like the flow of fuel through the diesel engine.  Flow ….  The boiler / firebox has to generate enough steam to make up for the steam used by the cylinders as the higher the speed, the more flow and steam is needed (basically). With valve cut off, steam flow becomes economized so it is not a direct proportion. But basically the boiler in the highest theoretical sense has to generate enough steam. And for a given top speed, if it can do that, then the limiting factor becomes the ability of that steam to flow through the cylinders.  Many later locomotives were so well designed that they could produce the steam, but using it is another matter. It has to be admitted, expanded and used, then exhausted. If the steam ports or admissions and exhaust ports are inadequate then these factors will limit the top speed; again, nothing to do with using a higher boiler pressure.
 
So to sum it up as related to the OP’s question, adequate boiler pressure is a factor in order to move tonnage (tractive effort) and boiler pressure is also a factor in efficiency (fuel burned and water used) but not a direct factor related to speed.
 
Nominal steam locomotive pressures of later day steam are probably in the 200 psi to 275 psi range, generally depending on the specific era and type of power as a baseline in understanding normal pressures.
 
Switching to a quick aside related to diesels:
 
A little SW1 (600HP) diesel can tie onto a big cut and move it … slowly due to the tractive effort which on the diesel is a direct relation to the weight on drivers, but still generally in the vicinity of 25% (due in part to the coefficient of  friction of steel wheel on steel rail and the wheel slip system). More horsepower will give you more speed, so a GP40-2 can pull it faster. More pointedly maybe, a GP38-2 (2000 hp) and a GP40-2 (3000 hp) have about the same tractive effort if geared and weigh the same. They can pull equally as much but the GP40-2 will pull it faster. So when doing tonnage ratings if you only have for example  a 25 mph railroad you don’t need the horsepower to pull the load, you only need the extra horsepower to move that same load at a higher speed. I’ve had a lot of operational guys tell me they needed horsepower to pull the tonnage up the grade. What they needed was tractive effort.  I’ve also heard fans say “Wow !!, that locomotive is pulling more tonnage than it is rated for !!” Well, no ….. as you don’t rate an engine to stall speed, you determine a minimum speed you desire for the grade or profile for that rating. So if the train is over rated tonnage it may only go 14 mph instead of 18 mph. It doesn’t mean it is pulling more tonnage than it is rated for before it will stall. That is a whole other conversation. As related to horsepower, that is a measurement of the rate at which work is done. more horsepower gets the work done faster.
 
Anyway, trying to compare steam with diesel power is like trying to compare green beans to oranges.
 

I finally figured out what stroke has to do with tractive effort and it doesn't BUT what is does do is to give
a backwards calc into the lever arm distance on the drive wheel. It saves the calculator the trouble of
having to go measure the distance between the pin and the axle, smart people those old guys! Piston 
dead head postion at both ends divided by two gives you that number and you could get from the OEM
paperwork wtthout getting dirty or needing a step ladder.

Out 

Thanks all for the amazing answers, glad I asked!

Another interesting test I do when I'm looking at a steam locomotive program is to compare the 'rated' tractive effort with the cylinder, pressure, stroke calculations as Doug shows and the relatively simple effort of taking the weight on the drivers * 25%.    If that calculation is lower than the rated effort you likely have a 'slippery' locomotive, but a well-designed one will have those numbers surprisingly close when I've done the numbers.   

I have some nice video of 611 slipping out on the Strasburg at Goffs on damp rail just taking their normal consist from a dead stop.  I've never done that exercise on 611 but would be interesting.   Stories of it hitting 100+ in service  on the N&W are likely true.

In steam days it seemed all about adding tractive effort, pile on helpers to get it up the hill, and today, the magic formula seems to be hp/ton to get it above a crawl because AC has such higher adhesion factors (in ideal conditions) but can't make track speed.  

Another interesting factor is that fuel consumption at speed is another seemingly inverse relationship between steam and diesel, to the point where you're taking any excess diesel power off-line now, where getting steam to speed to get the draft going efficiently and lower the valve cutoff is just the opposite.

randgust Wrote:
----------------------

> Outbound from Erie to Bellevue [N&W] 611 handled the
> [about 18-car] train in stride, track speed 70mph...

> the return trip was being handled by two
> squeaky-clean, factory fresh C30-7's ....
> We managed to hold a fairly constant 40-45,
> but those two units NEVER got it up to track speed.

If they're geared for it, and they're producing their
rated power, the two units would do 80+ mph on the
level with 18 cars. (As would the 611.)

Hey eljay! do you understand why this is wrong?

>  Full horsepower can be developed at zero mph if the wheels do not slip.

If you don't, you need to learn that first, or
you'll never understand what's going on.

​(No doubt train1275 knows better than that --
he just didn't express himself right.)



Edited 1 time(s). Last edit at 11/04/25 09:29 by timz.

The formula I quoted is usually used for electric motor apps but a steam locomotive drivers do rotate.
The N in the formula is rotating speed, zero mph calculates out to zero hp. You might have huge torque
potential (which is why drivers slip at low speeds) but the horsepower is zilch.

Out 

To clarify my point about horsepower, I am referring to diesel engine prime mover horsepower in that you can wind the throttle out to Notch 8 and ramp the engine speed up to full RPM at nominally a stall (yes, not really full HP as the load regulator / wheel slip wouldn't allow that). My point in comparison is that the diesel engine prime mover horsepower generation is independent of the rotational speed of the wheels. Brake horsepower vs. Drawbar HP.



Edited 2 time(s). Last edit at 11/04/25 15:52 by train1275.

A lot of that gets to regulation. You can wind that diesel prime mover up and regulate the power applied
to the traction motors right up to the point of wheelslip. EMD used to quote the ACe units has having a
starting traction coefficient of over 40%. You don't get that regulation in a steamer, once you crack the
throttle you can come to full cylinder pressure very quickly and your regulation becomes on and off the
throttle until the wheels quit slipping.

Out