Topics

Calculating Motive Power Requirements


Nelson Moyer <ku0a@...>
 

I’d like to assign motive power based upon tonnage of freight trains. What is the relationship between h.p. and tonnage? Is there a formula used by yardmasters or roundhouse foreman? I know of at least one club that does this, and perhaps some other operators do this as well.

 

Nelson Moyer


Bruce Smith
 

Nelson,

This information is typically found in the Employee Time Table of the prototype.  Since it completely depends on the ruling grade, it varies for each segment of a railroad.

Regards
Bruce Smith
Auburn, AL


From: STMFC@... [STMFC@...]
Sent: Thursday, August 13, 2015 7:52 PM
To: STMFC@...
Subject: [STMFC] Calculating Motive Power Requirements



I’d like to assign motive power based upon tonnage of freight trains. What is the relationship between h.p. and tonnage? Is there a formula used by yardmasters or roundhouse foreman? I know of at least one club that does this, and perhaps some other operators do this as well.

 

Nelson Moyer




Charles Peck
 

Big number of variables, Nelson.  Are you looking at reefers with fresh produce rushing to market?
Hoppers full of low value minerals needing the most economical transport?  Great Plains?  Through
the mountains, not around them?  A river valley route?  How long are your sidings?  Lots of short
trains or wait and drag as many carloads as you can through the pass to harbor?   
Yes, there are formula that tell a real railroad that X tons at X miles per hour up X ruling grade will 
need Y horsepower on dry rail. 
That formula fails on models.  You have to look at the profile (real or imagined), what sort of traffic
you choose for your road, and create a realistic loading.  Are you running 10 or 12 car trains?  Four
high horsepower units will look out of place. Unless, of course, you are modeling Saluda.  Running 
thirty cars and want it to look like sixty cars through Tennessee Pass?  OK, three or four units might
look OK there.  
Pick your biggest train and figure what looks right on it.  Then scale down from there.
One mans ideas, not a rule in any sense.
Chuck Peck in FL

On Thu, Aug 13, 2015 at 8:52 PM, 'Nelson Moyer' ku0a@... [STMFC] <STMFC@...> wrote:
 

I’d like to assign motive power based upon tonnage of freight trains. What is the relationship between h.p. and tonnage? Is there a formula used by yardmasters or roundhouse foreman? I know of at least one club that does this, and perhaps some other operators do this as well.

 

Nelson Moyer



Tim O'Connor
 


  > I'd like to assign motive power based upon tonnage of freight trains.

Grades are a huge consideration --

Rules of thumb --

    5 hp/ton on 0% grade (flat land)
   25 hp/ton on 1% grade
   45 hp/ton on 2% grade
   65 hp/ton of 3% grade

Also remember it takes an equal amount of BRAKING POWER to hold back
a train on a downgrade. This is why dynamic brakes were rapidly adopted
and why dieselization was so swift on steep mountain grades. A diesel
powered train with dynamics can control the train speed with very limited
use of train brakes.

You can make far more complicated calculations, of course. Steam locos
reach peak HP at mid-range speeds (30-40 mph) while diesels can maintain
constant HP. Normal DC traction motors couldn't handle slower speeds than
10 mph for very long or they would overheat. Steam booster trucks worked
only below 20 mph I think. Also, cold freight car bearings have higher
resistance than warm bearings.

And don't get Mike started on Tractive Effort! :-)

Tim O'


np328
 

I agree with Bruce on his statements regarding the employees timetables.  I believe that the dispatcher has the final say on signing off on the power assigned.  The wind and temperature are also a factor. The dispatchers train sheets have blanks for this data that is typically filled in.

Nelson, for more data look for the website/blogsite Tales from the Krug

 





It has some interesting data however out of respect for the sheriff, I will note the date frame of the site well post dates this sites 1960 cut off. Al Krug is an interesting railroader and has put some useful data on the site.  
                                                                                                       Jim Dick - St. Paul, MN


Nelson Moyer <ku0a@...>
 

Thanks for the link to Al Klug's site. I'll play with the calculator to see how it might be used in model railroad operation.

Nelson Moyer


On Aug 15, 2015, at 12:41 AM, "jcdworkingonthenp@... [STMFC]" <STMFC@...> wrote:

 

I agree with Bruce on his statements regarding the employees timetables.  I believe that the dispatcher has the final say on signing off on the power assigned.  The wind and temperature are also a factor. The dispatchers train sheets have blanks for this data that is typically filled in.

Nelson, for more data look for the website/blogsite Tales from the Krug

 





It has some interesting data however out of respect for the sheriff, I will note the date frame of the site well post dates this sites 1960 cut off. Al Krug is an interesting railroader and has put some useful data on the site.  
                                                                                                       Jim Dick - St. Paul, MN


Jim Betz
 

Nelson,

Often using any kind of "prototypical practice/methods" results in calculations
that don't really "work" for the layout. At the very minimum you will have to
make some decisions that "map" the horsepower in the locos to the trains you
are able to run on your layout - because, among other reasons, the prototype
ratings don't apply directly to our model trains.
I'm saying that using the same formulas that were used on the prototype
often results in 'errors' when applied to our layouts.

****

Another approach is to do -actual- motive power measurements ... on your layout.
This is easily done by first finding the ruling grade and then pulling a 'typical' train
up that grade and changing the number of cars until the loco handles it (with? or
without? any slipping is up to you/the layout owner).
Finally - create "locomotive cards" that are the same size as your car cards and
include the rating/pulling power for the loco. It helps if all of the cars on the
layout are the same weight (for the same size car). Using a rating that is right
on the edge of being the max the loco can carry may result in "emergency calls
for a helper" which although fun can seriously affect the 'flow' of the Op (so
decide whether or not you want this to happen and set your ratings based upon
that decision). The loco card "travels with the loco/train" just like the car cards -
so it is always available to anyone (the YM/hostler?) who is making the decisions
about what power to use on the trains. (Many layouts also have "caboose cards".)
Many layouts include specifying the locos that will be used on trains that are
made up during the session in the 'packets of stuff' that are used to assign the
trains to operators and/or given to the yards to use to make up the train(s).

Most layouts do not have much capability for/include in the operations the
ability to add helpers for "Just The Grade(s)" ... and so the motive power assigned
to the train is done in the originating yard or staging.
With this approach what you do is to "assign enough power for the train to get
itself over the RR". And, usually, the layout owner decides to not specify different
power requirements for different directions (even if that is possible) - if for no
other reason than to keep the number of locos 'balanced' on the ends/in the
yards. But some Ops include "light power moves" to re-balance. *G*

****

I operate on a lot of different layouts. One of them has a helper district and
many of the trains truly -require- a helper in order to get up the grade. They
won't make it without one. The way the Op works is that the train pulls up to
the departure point in the yard/town, the helper is added (and at the correct
placement/specified place in the train), and then clearance to proceed is gotten.
At the top of the grade the helpers are cut off and the train proceeds without
them. This is an "all steam" Ops (OK, only mostly steam) and so each locomotive
has its own crew/engineer (no consisting with the head end).

****

Helper operations can be/are a lot of fun - but in order to do them
effectively you may have to add Ops jobs such as "hostler" and "Helper
Engineer(s)" ... and you also have to allow time in the schedule to add/remove
helpers. Luckily, it is usually fairly easy to find a window where a helper/helper
set can be returned back down the hill - where it goes into the engine facility
to wait for its next assignment. *Great Stuff!!!*
- Jim B.


Charles Peck
 

You might also keep in mind that your "ruling grade" might not needfully be in
sight on your layout. It might be over the horizon towards the next division point.
Chuck Peck

On Sat, Aug 15, 2015 at 10:59 AM, jimbetz jimbetz@... [STMFC] <STMFC@...> wrote:
Nelson,

   Often using any kind of "prototypical practice/methods" results in
calculations
that don't really "work" for the layout.  At the very minimum you will
have to
make some decisions that "map" the horsepower in the locos to the trains you
are able to run on your layout - because, among other reasons, the prototype
ratings don't apply directly to our model trains.
   I'm saying that using the same formulas that were used on the prototype
often results in 'errors' when applied to our layouts.

                                                               ****

   Another approach is to do -actual- motive power measurements ... on
your layout.
This is easily done by first finding the ruling grade and then pulling a
'typical' train
up that grade and changing the number of cars until the loco handles it
(with? or
without? any slipping is up to you/the layout owner).
   Finally - create "locomotive cards" that are the same size as your
car cards and
include the rating/pulling power for the loco.  It helps if all of the
cars on the
layout are the same weight (for the same size car).  Using a rating that
is right
on the edge of being the max the loco can carry may result in "emergency
calls
for a helper" which although fun can seriously affect the 'flow' of the
Op (so
decide whether or not you want this to happen and set your ratings based
upon
that decision).  The loco card "travels with the loco/train" just like
the car cards -
so it is always available to anyone (the YM/hostler?) who is making the
decisions
about what power to use on the trains.  (Many layouts also have "caboose
cards".)
   Many layouts include specifying the locos that will be used on trains
that are
made up during the session in the 'packets of stuff' that are used to
assign the
trains to operators and/or given to the yards to use to make up the
train(s).

   Most layouts do not have much capability for/include in the
operations the
ability to add helpers for "Just The Grade(s)" ... and so the motive
power assigned
to the train is done in the originating yard or staging.
   With this approach what you do is to "assign enough power for the
train to get
itself over the RR".  And, usually, the layout owner decides to not
specify different
power requirements for different directions (even if that is possible) -
if for no
other reason than to keep the number of locos 'balanced' on the ends/in the
yards.  But some Ops include "light power moves" to re-balance.  *G*

                                                              ****

   I operate on a lot of different layouts.  One of them has a helper
district and
many of the trains truly -require- a helper in order to get up the
grade.  They
won't make it without one.  The way the Op works is that the train pulls
up to
the departure point in the yard/town, the helper is added (and at the
correct
placement/specified place in the train), and then clearance to proceed
is gotten.
At the top of the grade the helpers are cut off and the train proceeds
without
them.  This is an "all steam" Ops (OK, only mostly steam) and so each
locomotive
has its own crew/engineer (no consisting with the head end).

                                                            ****

   Helper operations can be/are a lot of fun - but in order to do them
effectively you may have to add Ops jobs such as "hostler" and "Helper
Engineer(s)" ... and you also have to allow time in the schedule to
add/remove
helpers.  Luckily, it is usually fairly easy to find a window where a
helper/helper
set can be returned back down the hill - where it goes into the engine
facility
to wait for its next assignment.  *Great Stuff!!!*
                                                           - Jim B.


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Nelson Moyer <ku0a@...>
 

I had another reply failure, so here’s the text of the failed message:

 

In my case, the ruling grade of 2% is in the helix, so it’s out of sight, over the horizon, over the rainbow, or whatever. I’ve already done pulling power on my locomotives on a straight 2% grade to get an idea of how many NMRA specification weighted cars they can pull. I don’t have the helix built yet, so I can’t repeat the tests on the helix curve.

 

I have the option of using helpers on the helix, or doubling the hill, which may be necessary when using steam power. There is a helper pocket at the bottom of the helix, and a passing track at the top of the helix.

 

Nelson Moyer

 


John Larkin
 

Remember that prototype tonnage ratings were done by first estimating how many tons a locomotive could pull over the assigned district, then often modifying the rating when experience dictated a change was necessary.  Some steam engines simply steamed and pulled better than anticipated and tonnage ratings would increase.  Thus the idea of just adding cars until the train stalls is essentially what was done on the prototype (yardmasters who needed to get rid of the last two cars on a track, for example)
and should work equally well on models, especially if there are somewhat uniform weights involved.

John Larkin



On Saturday, August 15, 2015 10:25 AM, "'Nelson Moyer' ku0a@... [STMFC]" wrote:


 
I had another reply failure, so here’s the text of the failed message:
 
In my case, the ruling grade of 2% is in the helix, so it’s out of sight, over the horizon, over the rainbow, or whatever. I’ve already done pulling power on my locomotives on a straight 2% grade to get an idea of how many NMRA specification weighted cars they can pull. I don’t have the helix built yet, so I can’t repeat the tests on the helix curve.
 
I have the option of using helpers on the helix, or doubling the hill, which may be necessary when using steam power. There is a helper pocket at the bottom of the helix, and a passing track at the top of the helix.
 
Nelson Moyer
 



devansprr
 

Prototype methods and basis for locomotive ratings and assignments appear to vary widely by railroad, division, and era.

I would strongly urge checking into your prototype's methods for the location and era of interest. I have not investigated widely, but several have researched the PRR's steam era practice and it was pretty sophisticated, especially on divisions where stalled trains could not be tolerated due to the backup and delay of other trains. I have not found any evidence that dispatchers had any input to locomotive selection on the PRR - I believe it was enforced by traffic managers and road foreman.

For each division and terminal pair the PRR would provide a tonnage rating for each class of locomotive, with further breakdowns for priority freights versus "mineral" extras. Some of the PRR's published books even report the location of the ruling grade and the minimum speed that the RR expected trains to sustain on that grade.

The PRR did have a "de-rating" scheme for low temperatures and wet rail, but not, as far as I know, for the wind. But I have read on the OpSig group that modern railroads crossing the midwest have found wind direction and speed to play havoc with the ratings on double-stacks (obviously past our era) so they do have correction factors for those conditions. Conversely, with modern roller bearings, the impact of temperature on train drag is now much less.

For the prototype, the total elevation change of the grade can be important, since the prototype "enjoys" the benefits of trading momentum for changes in elevation. For the prototype, the momentum at 60 miles an hour can be traded for a climb of over 100 feet.

Not so for the modeler, where, in HO, the momentum of 60 scale mph translates to a climb of only 3/8 of a real inch. Even worse in N-Scale (no way to scale gravity). This is why it is so important to avoid short lengths of steeper than ruling grade on model railroads. A long 2% grade with a few feet of 2.5% grade will actually have a ruling grade of 2.5%, not 2%.

Conversely the PRR had several 25 foot high flyovers on their main line where starting the flyover at track speed would knock less than 20 mph off the train's speed, without changing the throttle. Such flyovers could actually have pretty steep grades, since the momentum effect doesn't care about the grade, only the change in elevation. Unfortunately this effect is not available to modelers, although, based on recent news reports, an G scale layout on Pluto might have close to G Scale gravity, but not HO - Pluto is still much too big for HO scale gravity ;-)

For HO model railroads, in general steam locomotives can never match their prototype ratings if pulling cars weighted to the NMRA RP, while diesels can often pull more than their prototype cousins. Within the PRR modeling community, Bowser steam locomotives do have a reputation for breaking that general statement, while their Kato based F-units significantly outperform the prototypes..

I would note I have a good friend who has outstanding track work, and was weighted most of his fleet to only 2/3 to 3/4 of NMRA RP, yet still has very reliable operations. That, combined with very free rolling trucks, has enabled his HO steam locomotives to come closer to prototype performance. (Remember that a large prototype steam locomotive on a 2% grade might be rated at less than twenty 50 ton (capacity) cars)

And there has been debate over the years on the LDSig group concerning curve compensation and grades. Best to test model equipment when designing a layout, although for diesels, there is little to worry about. Steam is the exact opposite, as those who try to run long steam trains on steep grades usually find out. Lots of discussion on this topic in the past on the LDSig and OpSig Yahoo groups, which I would recommend for those designing STMFC era layouts, especially those who plan to run steam on the head-end.

Dave Evans



---In STMFC@..., <jcdworkingonthenp@...> wrote :

I agree with Bruce on his statements regarding the employees timetables.  I believe that the dispatcher has the final say on signing off on the power assigned.  The wind and temperature are also a factor. The dispatchers train sheets have blanks for this data that is typically filled in.

Nelson, for more data look for the website/blogsite Tales from the Krug

 





It has some interesting data however out of respect for the sheriff, I will note the date frame of the site well post dates this sites 1960 cut off. Al Krug is an interesting railroader and has put some useful data on the site.  
                                                                                                       Jim Dick - St. Paul, MN


Tim O'Connor
 

A long 2% grade with a few feet of 2.5% grade will actually have a
> ruling grade of 2.5%, not 2%.

Most railroads calculated �ruling grade compensated� to take into account
both curvature and the length of the "ruling grade". A very short 2.5% grade
(say 400 feet long) probably would have very little effect on the compensated
ruling grade, so that power requirements were based on the compensated number.

Tim O'Connor


devansprr
 

Tim,

The definition of "short", for the prototype, would be a function of train lengths and train speed - I don't think there is a generic number. It would also depend on whether the RR expected a crew to restart a stopped train (for signal or traffic ahead) on that steeper section (and whether the locomotives were steam or diesel.)

The point I was trying to make, specific to the original thread, is that the situation is much different on model railroads. A helix with circumference of 200 inches, designed to climb 12 inches in three turns, results in a 2% grade. But if the helix has the first two turns climb 3.5 inches each, and the final circle climbs 5 inches, the ruling grade will be 2.5%, not 2.0%. And if locomotives and train lengths are selected to barely climb the planned 2% grade, no amount of "running start" will get up the 2.5% portion of the helix, since without scaling gravity, there is no momentum effect for trading speed versus elevation - entering the 2.5% portion of the helix at 60 scale mph will not help the situation.

Even though each turn in that helix represents 1450 scale feet, for a high volume mainline railroad that dispatched 80 car trains with enough motive power to maintain track speed above 10 to 15 mph on ruling grades (as the PRR did in the steam era), over 1450 prototype feet, the difference of 0.5% is only 7.5 feet of additional climb, which can easily be overcome by momentum on the prototype, so the prototype could declare the ruling grade as 2%, especially since only part of the train would be on that steeper section at any instant (this is assuming no curves) . Conversely, for a branch line with small locos that would struggle (as in 5 mph) to climb a grade with only 8 cars in tow, a 400 foot section of grade at 2.5% WILL be the ruling grade.

But I think I hear the keys rattling, and I am really just trying to provide a warning to helix builders - if you plan to run trains long enough to severely challenge your locomotive's pulling power, then your helix needs to have a VERY uniform grade - and we are talking 1/2 and even 1/4 inch errors (in HO - half that in N-scale) can cause problems (which is why some helix builders use threaded rod, so they can adjust the grade more precisely.) Seek guidance on the LDSig and OpSig Yahoo groups for further information.

Dave Evans

---In STMFC@..., <timboconnor@...> wrote :

> A long 2% grade with a few feet of 2.5% grade will actually have a
> ruling grade of 2.5%, not 2%.

Most railroads calculated �ruling grade compensated� to take into account
both curvature and the length of the "ruling grade". A very short 2.5% grade
(say 400 feet long) probably would have very little effect on the compensated
ruling grade, so that power requirements were based on the compensated number.

Tim O'Connor


midrly
 

I hope that you'll not mind my being a little late with this.

On the railway where I work, 0.9 HPT is the formula used in both directions on 189 miles of train run.  Westward, we have a 0.9 % grade with about a half-mile-long piece of 1%.  Eastward, we have a grade of 0.9 %.  The westward ruling grade is the famous Copetown Hill, where one usually breaks out lunch while on the move.  Lots of time to eat it during the usual forty-minute or so climb over nine miles up the Niagara escarpment.

And AC power is allowed 0.6 HPT on this same run!

Note--I've had 1.3 HPT trains that roll like slugs, and 0.9 HPT trains that can roll along at 60 per almost all the way.  How a train rolls depends on train length, wind resistance, and wet rail as well as tonnage.  Get stopped somewhere, and one can have a hard time getting up to track speed again.

Steve Lucas