i recently ran a train on my layout. at least tried to. here's the initial setup : two kato es44ac with digitrax dn163k1c in front of 79 lbf/huberts johnston aeroflow coal cars. all hoppers have been weighted to 40 gramms each. total weight of the train is 3160 gramms (plus/minus a few grams). i have not been able to start this train as both locos were spinning. then i tried a different setup : 1 kato es44ac, 45 coal cars, 1 kato es44ac, 34 coal cars. same train, different location of one of the locos. this train did start and run smooth. almost no wheel spinning. i was able to stop/start the train thru s-curves without major traction problems. just a tiny bit of spinning. i probably could add a few more cars but then my siding would not be long enough. this really made me wonder. i remember an article in 'trains' describing these things. railroad companies calculate where a dpu unit has to be coupled in to be most efficient. didn't think this is also valid for n-scale. now we need to figure out how to calculate this stuff for our little trains.
With both locos up front, can you start the train if you back it up a bit, first, to bunch up the slack? Is this the Trains magazine article: http://www.getransportation.com/resources/doc_download/329-trains-magazine-locotrol-feature.html
My HS physics is coming back to haunt me. With two locomotives up front, part of your energy is spent pulling or pushing the second locomotive when it loses adhesion. It's like they don't load up together like one giant locomotive. When you spread your power, both locomotives are picking up smaller seperate but connected loads that they each can handle. This also helps prevent "closeline." I saw that article in Trains. The big railroads also have to take into account lateral forces on curves and fuel economy so they may put their DPUs when they have the best mix of power and fuel economy and/or train handling, not necessarily just the most power. To do the calculations for N scale would be and interesting project, though. Andy Tetsu Uma
backing up a bit doesn't help. as soon as the train is stretched again both units start to slip and yes, that is the article i referred to.
That only works if the cars are all body mounted couplers with very close speed matching on the locomotives.
My own "experiments / trial and error" Using DC Kato AC4400 or 2 Atlas SD60s up front Atlas geep about 2/3rds of the way back I placed lighter cars such as shorty beer can tank cars behind the geep and 40+ foot box cars in front of it. My Conclusion: This would increase the length of a train I could move. One test moved 16 cars up a 6% grade. It definitely was affected by how much followed the geep and how much was in front of it. The geep picked up "slack" which gave the lead locos what they needed. Notes - Caution!: "String line derailments" may result from improper "balancing" of units and cars. Do not ever test this if there is a risk of cars falling to the floor. My testing was geared more toward running a mid train DPU using DC. I was not focused on the increased pulling power by doing so. Part of the advantage of the Atlas geep is the light weight. It would slip if the lead units stalled out. Again, be very careful trying to duplicate this. My cars landed on soft carpet. Where are yours going?
I expect that even with DCC, (and I have no experience with it), that could lead to serious string lining. Again, just speculation and not based on evidence empirical or otherwise.
The ONLY train and lots of people have commented when they have seen it, a UFIX train it is 32 cars and has accumates. BUT on DC one SD70MAC on front one on back as DPU works. the couplers take the pressure of having a DP pushing even if it is faster than the lead. These have theonly accumates I have work, and never come apart... It is all a matter of working out wieght and length, just like the real railroads you have to think a little, it might just be trial and eror also. It is FUN to have a challenge and then have it actually be able to work.
I have done this kind of experiment on my layout as well, 2 SD90's on the head end have a hard time getting 100 Intermountain grain cars rolling, but put 1 at the front and 1 50 cars back and they can pull the entire train. (Not that I regularly run 100 car trains on my layout, but it is fun to do sometimes!) I remember running 110 cars when Sandro was visiting a couple of years ago, again, pulled by 2 SD90's!
it does work. all my cars have TRUCK mounted couplers. one es44ac on each end easily moves the train. i did run it for more than 1 hour without any issues. no derailments. oh, two more ore less speed matched engines , several turnouts, crossing different booster sections (different boosters as well).
Just from my own experiance with a 90 car train of trainworx quad hoppers. Get rid of the weight so your engines can move the thing, my trainworx cars have no metal weight in them at all, maybe a little in the first 3 or 5 cars but that's it. Next, teach your self to be a good engineer at both the front and rear of your train, you're going to have to make independent adjustments at both the front and rear at different times if you have grades. When cresting a grade, step it back a number or two on the head end just after it crests, watch your slack, ease up on the rear units to match the head end as they near the crest, maybe even a hair slower than the head end as they go down hill. Stuff like that you'll just have to play with and figure out for your layout. On flat terrain and getting started, start pushing easy on the rear first and the head end should have an easier time getting started. While moving, keep some slack bunched at the rear, thats your cusion if a rear unit momentarly stalls or something. Obviously if your lead units can't really pull the whole train your rear units better be doing something and you'll have slack back there. For N-scale purposes your lead units should probably be able to pull the whole train by themselves. For some reason model trains don't like to play like the real ones, so, creating a real situation where rear units actually have to shove for the train to move may cause more problems than you want. On MRL on the Bozeman Pass, we cut are helpers in the middle+the differance of front and DP powered axles. So, on a loaded 120 car coal train with 3 ES44AC's (8 powered axles x3 =24) up front and one ES44AC (8 powered axles) DP on the rear, there's 8 less powered axles on the rear. We go further back in the train one car for every less powered axle on the rear. So on the train above, 120cars/2=60 is the middle + 8 (one car for each powered axle differance), the helpers would go 68 cars deep from the head end on this train.
that's a cool formula to calculate the dpu position. the rest of your post is quite interesting as it is completely different from my experience. all my coal hoppers are really heavy, weighted at 40 grams which is quite a bit above nmra if i remember correctly. also i don't run the units independently but mu-ed using dcc. i can run the coal train with one unit on each end or one in front, one after 2/3. both works fine. (none of the units is able to run the train by itself) i just apply power and off we go. as said i've been running the train for one hour non-stop. no issues with stalling or similar. but then, my layout is completely flat. curves have a minimum radius of 25". there's a 'double horseshoe' curve. first one is approx 240 degrees followed by 150 degrees in the opposite direction. 25" radius.
