Northwestern Pacific Railroad Network

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My April Fools status sparked a little discussion on train tonnage and motive power calculations:

Simply put, there is a lot that goes into deciding how many units are assigned to a train. Grade, curvature, AC or DC locomotives, tractive effort etc. are just some of the many factors. There are formulas that we use to determine these things, but there is a different formula for each grade, locomotive type, powered axle rating, etc. far too many to list here. Powered axle rating is different from the physical number of axles on a unit, for example the six axle ES44AC is rated at 12.5 powered axles, Dash 9 are rated at 8 powered axles etc. There are alos maximum limits on headend powered axles and total (including DPU) powered axles. 42 max on all trains except intermodal, which can have 48.

Dynamic braking also plays a big part. Often times, the number of units that can be used for dynamic braking is less than what can be used to power the train. For example BNSF limits trains to 28 powered axles except on solid double stacks, which can have 32. A manifest with four Dash 9's for example can use all four uphill, but only three for dynamic braking, since Dash 9's are rated at 8 powered axles. Note the limit is per CONSIST, so trains with DPU sets can have more than 28 powered axles, as long as no more than that is on each power set. There are certain restrictions regarding DPU sets on the rear versus those mid-train. A train with six units in a 3x3 setup for example can have use all axles for dynamics only if the DPU set was entrained. It would be against the rules assuming the DPU's were on the rear.

ABTH 102.12.6 for helper/DP placement

Distributed Power/Helper Limitations and Placement
1. Rated Powered Axle Limitations
Unless individual subdivisions special instructions specify otherwise, the following rated powered axle
(RPA) limitations apply to each DP/helper consists:
(Use Locomotive Data Table to determine powered axle ratings)
· All trains (excluding unit bulk commodity trains) - 16 RPA or less on rear and 24 RPA, cut in.
· Solid, empty unit bulk commodity trains - 12 RPA or less on rear or cut in.
· Solid, loaded unit bulk commodity trains - 24 RPA or less on rear or cut in. (bulk commodity trains =
coal, grain, potash, taconite, molten sulphur, etc.)
2. Maximum power differential allowed.
On trains operating with a single DP/helper consist, the total operative locomotives of the lead consist must
not exceed the remote DP/Helper consist by more than 16 rated powered axles.
3. DP/Helper Train make Up Restrictions
a. All DP/helpers locomotives must be placed ahead of:
1. All TTOX (single unit 2 axle equipment) and TTFX (4-unit solid drawbar connected 2-axle per unit
equipment), regardless of weight.
2. 2-axle scale test cars.
3. Cars designated “Rear end only”.
4. Occupied caboose.
5. Multiplatform (2 unit), solid drawbar-connected gondola cars with initials AMGX.
b. DP/Helper locomotives consists rated at 8 powered axles or less are exempt from DP/Helper train
make up instructions outlined in Item 3 below.
c. Except for empty unit bulk commodity trains, which may be operated with 12 rated powered axles,
when DP/helper locomotive consist powered axle rating is between 9 and 24 powered axles, the
following cars may not be placed:
· Within 10 cars/platforms AHEAD a DP/helper consist if on rear of train, or
· Within 5 cars/platforms AHEAD a cut in DP/helper consist.
1. A conventional car (non multi-platform) weighing less than 45 tons.
2. A multi-platform car having a single empty platform.
3. A car 45 feet or less in length coupled to a car 80 feet or longer in length.
Note: Does not apply to multi-platform cars except those with individual platforms exceeding 80 ft. in length.
Examples: Twin flat cars and AutoMax cars.
4. Any 80 foot or longer flat car with a single trailer/container, regardless of weight.
Note: This includes twin flat cars (solid-drawbar connected flat cars, TTEX and RTTX series) with a single
trailer/container on either platform.

For Powered Axles, rather than using total powered axles on line, we mostly use HPT to determine how many units to have online:

102.11 Powered Axle Limitation
Locomotive consists must not have in excess of 42 equivalent powered axles. Excess axles of power
must be isolated. Unless otherwise restricted, trains made up entirely of intermodal equipment may
operate with a maximum of 48 equivalent powered axles.

106.1 Regulating Horsepower per Ton
Train and engine crews are required to isolate or shut down units in a consist that are in excess of the
scheduled Horsepower per Ton (HPT) as identified on the train list and train profile. When train list or profile
does not indicate scheduled HPT, the train dispatcher may advise crew of train’s scheduled HPT. Unless
otherwise outlined below, crews must isolate or shut down excess units, but not more than 0.5 HPT below
scheduled HPT:
Exception: On trains with symbols beginning in “Z”, “Q” or “P”, isolate or shut down excess units to as
close to but not below scheduled HPT. All intermodal, manifest trains and loaded bulk commodity trains
operated with distributed power are exempt from HPT limitations for fuel conservation.
1.Trains operating on steep grades of the subdivisions defined below, may use all available horsepower:
Cajon, Gateway, Glorieta, Hi Line, Mojave, Pike’s Peak, Raton, Scenic and Stampede subdivisions.
After train has traversed steep grade, excess units must again be isolated/shut down as soon as safety
2.Locomotive is utilized in dynamic brake mode only by utilizing “Dynamic Brake Only” position on
Isolation Switch, if available. Dynamic brake axle limitations still apply.
3.Do not isolate, shut down or utilize Dynamic Brake Only feature on a unit for fuel conservation if it
causes your train to exceed 400 tons per operative axle of dynamic brake (TODB).
4. Empty unit trains, i.e., coal, taconite, grain, potash and sulphur must not operate with more than 9,000
working horsepower (HP) on line.
5.Train dispatcher authorizes train and engine crews to place excess locomotives on line.
Note: A crew member at a train’s originating terminal must report any locomotive(s) isolated, shut down or
placed in Dynamic Brake Only Mode for fuel conservation as outlined above to the NOC Mechanical Desk.
In addition, on en route trains, a crew member must report changes in status of any locomotive(s) being
utilized in power due to change in train’s HPT or en route changes to locomotive consist. Locomotive(s)
within your consist equipped with Automatic Engine Start/Stop Systems (AESS) do not relieve you from
the responsibility of reducing horsepower as outlined in this rule. When selecting locomotives to shut down
or isolated in compliance with this rule, locomotives not equipped with AESS should be shut down or
isolated as required first before selecting locomotives equipped with AESS

For dynamic braking:

103.2.1 Dynamic Brake Limitations
High buff force generated by dynamic brake retarding force may cause a derailment or damage the
track structure. Therefore, limit dynamic brake retarding force as follows:
1.Limit the total operative dynamic brake to 28 equivalent dynamic brake axles unless further restricted
by another rule or special instruction.
Exception: Trains with remote and/or manned helper locomotive consists entrained or at the rear of
the train may have the maximum allowable dynamic brake axles for each locomotive consist placed
within the train.
2.Limit the dynamic brake retarding force by cutting out the dynamic brake on the trailing locomotive(s)
using the dynamic brake cutout switch or the dynamic brake selector switch on the control panel.
3.The preferred option is to cut out the basic dynamic brake(s) on a trailing locomotive(s).
4.When approaching and operating through turnouts or disturbed track areas with train’s air brakes
released, use the dynamic brake handle position to limit retarding force to 50 percent of maximum
(dynamic brake handle position number 4). Continue to limit the braking effort until at least half the
train has passed the restricted area. At speeds of 10 MPH or less, this limitation applies only if 12
axles or more of extended range dynamic brakes are being utilized.

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Replies to This Discussion

Oh, I got this etched deep now!! LOL   All I can say is that I really did know the railroads have this all figured out. I still like your formula for horsepower per ton x12  divided by grade etc. Although general it does kinda of help with NWP and guessing when they might need more than one unit. Engineering is complex.  Simple tire size on a vehicle is by engineering (weight, horsepower, ground pressure per inch of tread, conditions) and thats why most people don't really know what they are doing when they change a tire size. Point is, the more you learn about something, the more you realize you don't know. Thank you very much for taking the time to post.

Just thought this was interesting since people here like to reminisce about the days of 8-10 SD9's tackling Ridge Hill with a monster train. Two, yes two ES44AC's can pull an 85 car 7,000 ton freight south over Ridge and maintain about 13 mph. The drawbar max for Ridge is 4700 tons, so one would have to run as a DPU, but two is all you would need. Going north up the 3% grade hey would make about 8 mph. I'd throw in a third unit going north if I were in charge, in a 2x1 configuration.

Only with poor memory from years back do I think I heard that there was a concern of pulling a heavy train apart and also could occur as the head end was going down while the rear was going up. That could be more so if the engineer applied too much power on the down side. I would guess that is why mid or rear end helpers and also carefull where to put helpers that are added for dynamic braking.  Seriously, pretty complex and way beyond anything I know.  Once again, thanks

That's why you put the DPU's in asynchronous control when cresting a hill. Have them keep applying power while the head end units are in dynamics.


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