I model BNSF in N scale and am looking for opinions as to whether I should run DPU and helpers or just DPU. The mountain (Highiline) subdivision runs eastbound for 421/2 feet of 2.54 percent grade to the summit of the mountain. Down hill from summit is 481/2 feet of 2.24 percent grade. I will be running heay trains of greater than 30 cars and over 960 grams (over 34 ounces). I do have an easier grade around the mountain that is double track that begins about half way up the mountain and bypasses the summit. It is a ,73 percent grade of 371/2 feet. Since I am running a modern railraod and DPU has become the norm. I am running up long steep grades would helpers be warented or just have 2+2 or 3+2 DPU engines for my trains. Helpers make for additional interest and running, but are they appropriate for my railroad.
Do understand correctly, you want to run head end, mid train, and rear units? The last two as helpers or as DPU?
I don’t have personal experience using DPUs/helpers. But what I’ve heard they work best and safest if they are really needed to get the train up the hill.
In model railroading the only difference between manned helpers and DPUs seems to be that helpers are cut in and out somewhere on the road.
On the prototype it is an economical question. If the hill is long and steep enough compared to the run the go with DPU, is it comparatively short they might run helpers. And don’t forget dynamic braking downhill.
In times of DPU there are not many helper districts left. CSX’s Sand Patch, NS out of Altoona, and MRL’s Mullan Pass. As far as I know BNSF and UP us manned helpers on a very limited basis on Cajon Pass and a few other locations. Usually DPUs do the job.
Regards, Volker
Edit: BNSF has power limitations for its trains. Trains are only allowed to have a defined number of “rated powered axles” (RTA): www.smartlocal933.org/BNSF%20SSI%20No.4.pdf
Look at page 70. The RTA for different locomotives can be found on page 7
To model helpers get a friend with a throttle to run the second set of motive power, you have to actually be in sync with each other like they did in the old days. If you have a grade to justify it do it!
On Cajon Pass the Santa Fe has several locomotives on the head end of each train. For most of the climb that is enough but when the east bound trains reach the steepest part of the hill which is right before the summit they add pushers to the very end of the train to help. The same set of pushers then backs down the pass and pushes the next train. West bound trains don’t require pushers because the grade is not as steep.
Even though the grades are 2.54 and 2,24 from the base of the mountain to the summit, I do have short sections of track as steep as 2.93 percent. Would I use that when determining which engines based on RTA for a particular train or would I use the 2,54 or 2.24 grades and add an extra engine to the train?
That must be for main 3, the original track. The main 3 has a 3% grade from Cajon to Summit. All other track has 2.2% grade. If help is needed on these tracks it is needed from San Bernadino. Trains had an interesting map of the month: http://trn.trains.com/~/media/files/pdf/map-of-the-month/trnm0811_acajonpass.pdf
Regards, Volker
Usually the ruling grade on a subdivision determines the power need.
In former years railroads had tonnage ratings for each locomotive on a special route. For example SD45s had a rating of 1,275 tons from Colfax to Sparks across Donner Pass. So a Train of 5,000 tons needed 4 SD45.
Today each railroad uses a slightly different method mostly based on rated power axle (RPA) or equivalent power axle (EPA). For each subdivision/mountain pass is determined how many RPA or EPA a type of train needs for 1,000 tons or how many tons 1 RPA or 1 ETA can move. Sorry I haven’t found information about details.
Perhaps you could make up a table determining how many cars a single axle can move on you model railroad and adjust it to the axle limits (or make your own).
Regards, Volker
What constitutes a ruling grade. the total grade from level to the highest point (on my railroad for example 2.54 percent) or the highest grade, even for a short stretch of track (In my case 2.93 percent)?
The ruling grade eeds the largest tractive or braking effort on a subdivision. It is not necessarily the steepest grade. When the steepest grade is shorter than the trains part of them is on level or less steep grade. A less steep grade longer than the trains then might become the ruling grade.
In your case the 2.93% might not the ruling grade if the trains are longer than the grade. It isn’t the lesser grade before the 2.93% either as a train can be partly on both grades at the same time. It would be somewhere in-between.
In case the 2.93% grade is longer than your trains it is the ruling grade.
In helper times railroads would often declare your lesser grade as ruling grade and add helpers for the 2.93%.
Regards, Volker
I think it really depends on the era and route. Horseshoe Curve famously still uses helpers. However in comparison, Soldier Summit on the old DRGW is almost entirely DPU units now with no recent manned helper runs to my knowledge.
Of course you could have manned rescue helper moves (which are kind of common in the modern age were power desks often fail to assign enough DPU’s to a heavy train passing through mountain territory). Set up trains to heavy and without enough power to pull up the grade, and once up on the grade and the train stalls out; have the crew call the nearest terminal for a manned helper. Then shove everything else on line into sidings to wait for the light engine set being ran by the second crew to run out from the terminal to the stalled train. Or if there is a local switching crew nearby have them abandon their local train in a siding and relocate their engines to helping the underpowered train. I know its not good practice to intentionally set up underpowered trains, but it does imitate a common ocurance on underpowered trains in the mountainous west in the current day and age.
Lastly clearing the line for the light helper rescue move to reach the struggling train will wreck havoc on operating schedules for the session (especially if passenger trains are involved); and keep the other crews who have to wait for the rescue to pass on the line; extending the operator’s time on the layout and providing a ‘surprise’ extra for another crew to run.
Occasionally having an undrpowered train is an interesting idea. I will have to keep that in mind. My mainline is fully double tracked. The Valley subdivision has a short grade of 1.67 percent east bound. At the hump yard is where I will add the additional power to climb over the mountain. West bound I do not have a good place to add additional power, so I will have all power on the train at staging. I have a pretty good tonnage rating system which I will use, since I do not fully underatand how to compute an RPA. Does anyone have any ideas on how to do this?
The tonnage rating system is good enough. SP got along with tonnage per locomotive type for many years.
The prototype calculates the restance of a train depending on grade, curvature, bearing, rolling, air resistance. The Davis formula comes to mind.
Union Pacific has defined 1 “equivalent power axle” (EPA) as 10,000 lbs tractive effort (TE). Knowing the resistance you can devide by 10,000 lbs TE and get the required number of EPAs. The UP System Special Instruction page 32 contains the EPA per locomotive type: https://www.up.com/ert/ssi.pdf
In the 1980s SD45 and SD40 had almost the same tonnage rating across Donner Pass. Now you have different wheel slip software, different adhesion factors between DC and AC locomotives and that shows in the EPA per locomotive.
For model railroading purposes a tonnage rating per locomotive for different grades is good enough, I think.
Regards, Volker
My tonnage rating system works out pretty accurately for model railroad purposes. You take the tractiv effort of a locomotive, divide it by 160 for N scale to get the Tonnage rating on level track. Divide that number by the grade and you get a tonnage rating for that grade. For example. For an SD40-2 the Continuous tractive effort is 82,100 oiunds. Divide that 160 gives you 513.125 or 513 tons on flat andlevel tracl. Divide that by the grade, for exampe 2.93 percent and you get 175.127986348 opr 175 tons. I use grams of weight and use the actual weight of each car, so 175 grams will enable the engine to pull that much weight up the grade. A 500 gram train would require 4 SD40-2’s to negotiaste the grade.
Your formula gives funny results for grades of 1% or less. For a 1% grade, you would get the same 513 tons you give for the level track. For a 0.1% grade, you would get an astonishing 5130 tons. You might also want to consider that a level track has formally a 0% grade, and the result of a division by zero is undefined.