# Vertical separation of track over track (HO)



## ajay (Jan 24, 2014)

Hi everyone. I'm still finalizing my HO track plan & there's a spot where a bridge will carry one track over another. I wish to minimize the vertical separation to avoid steep grades in the vicinity, since the tracks have to meet up again. Right now I've got it down to just over 4" railhead to railhead at the crossing point. I'll use whatever kind of bridge works best. Should 4" be enough? Are there special bridges for this?


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## MichaelE (Mar 7, 2018)

Four inches is plenty. That is well over NMRA standards.


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## JerryH (Nov 18, 2012)

3 inches from the railhead to the lowest overhead obstruction. The thickness of the deck comes into play with your 4 inch figure.


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## mesenteria (Oct 29, 2015)

Can my car fit under the bridge?

Answer - Which car...…...under which bridge? Both are factors.

Measure the height of the highest point of any rolling stock from the tire surface near the flange to the tip of the vehicle. That, plus 1/8", is your effective clearance. Above that height, you need to build a suitable structure to hold the road-bed, ballast, ties, rails, AND the borne tonnages passing over them all. Ostensibly, you could possibly get away with using some short popsicle sticks or tongue depressors as the structure for a short distance, although it wouldn't come close to the correct prototypical engineering requirements. But for our models, with say a span of about 3", enough for the gauge loading for a perpendicular underpass, they would suffice if you really want to minimize the grade to and from the overpass height.


If, on the other hand, you would rather use a scale prototypical structure, you will need more than just the 1/8" or so for insurance above that highest point we measured earlier. You need the thickness of the 'engineered' supporting structure.


BTW, don't forget to include the lengths of four vertical curves, two on either side of the overpass, especially if what's left between them has to be that much steeper to get to the correct height.


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## ajay (Jan 24, 2014)

Thanks!


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## mopac (Feb 24, 2011)

Not trying to throw a wrench in your plan, just want to make sure you are aware.
Your plan is what I call an up and over. I had one on my first layout. It did not work
out well. Here is why. Your rise should be 1/4" per 1 foot. To rise 4" and then you have to come down. At 1/4" per foot It will take 16 Ft up and 16 Ft down. A lot of real estate. I was on a 4X8. I did not have the room.
That is to have a 2% rise and fall. If you have room go for it.


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## gregc (Apr 25, 2015)




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## Gramps (Feb 28, 2016)

mopac said:


> Not trying to throw a wrench in your plan, just want to make sure you are aware.
> Your plan is what I call an up and over. I had one on my first layout. It did not work
> out well. Here is why. Your rise should be 1/4" per 1 foot. To rise 4" and then you have to come down. At 1/4" per foot It will take 16 Ft up and 16 Ft down. A lot of real estate. I was on a 4X8. I did not have the room.
> That is to have a 2% rise and fall. If you have room go for it.


This advice is well worth repeating.


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## CTValleyRR (Jul 26, 2014)

mopac said:


> Not trying to throw a wrench in your plan, just want to make sure you are aware.
> Your plan is what I call an up and over. I had one on my first layout. It did not work
> out well. Here is why. Your rise should be 1/4" per 1 foot. To rise 4" and then you have to come down. At 1/4" per foot It will take 16 Ft up and 16 Ft down. A lot of real estate. I was on a 4X8. I did not have the room.
> That is to have a 2% rise and fall. If you have room go for it.


I agree with Gramps about this advice. 3" to the lowest overhead obstruction is plenty of room for all but the tallest equipment. 

But that's not really the issue -- as mopac points out, it's the necessary space to get up and over with reasonable grades that is what you really need to worry about. Beginners often overlook this, and end up with a track plan that looks great on paper but is unusable (this includes me, on the first layout I built, 40-ish years ago).


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## MtRR75 (Nov 27, 2013)

Keep in mind that, depending on your layout design, you might be able to have the lower track go downhill as it approaches bridge, then rise back up on the other side. This will reduce the amount of rise needed for the upper track.


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## Vincent (Jan 28, 2018)

Somewhere, I saw an ad for an HO figure 8 bridge. The whole thing was six feet long; not six feet of track--it was six feet from end to end.


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## gregc (Apr 25, 2015)

A common model railroad design element is a 270 degree loop intersecting itself at an overpass. Minimum car clearance is typically 2.75 in. But material between the clearance height and the railheads above must also be considered. The bare minimum would be tie thickness. In addtion, the effective grade of the curve should also be considered.

The following table reports the true grade (%), rise over run, and effective grades (%) for material plus tie thickness and for a curve radii from 18 - 40 in., using the following equations:

distance	=	radius * 2 * (1 + π * 270 / 360)
grade	=	(2.75 + thickness) / distance
effective grade	=	grade + 32 / radius​
Radius (in.) vs Grade and Effective Grade

```
Thickness     18        22        26        30        32        34        36        38        40    "
    0.25 "   2.5 4.3   2.1 3.5   1.8 3.0   1.5 2.6   1.4 2.4   1.4 2.3   1.3 2.2   1.2 2.1   1.2 2.0 %
    0.50 "   2.7 4.5   2.3 3.7   1.9 3.1   1.7 2.7   1.6 2.6   1.5 2.4   1.4 2.3   1.3 2.2   1.3 2.1 %
    0.75 "   2.9 4.7   2.4 3.9   2.1 3.3   1.8 2.9   1.7 2.7   1.6 2.5   1.5 2.4   1.4 2.3   1.4 2.2 %
    1.00 "   3.2 4.9   2.6 4.0   2.2 3.4   1.9 3.0   1.8 2.8   1.7 2.6   1.6 2.5   1.5 2.4   1.4 2.2 %
```
It is surprising how large a radius is required (e.g. 40") for desirable grade of 2% or less. For each thickness increase of 1/4", the minimum radius resulting in a 2% grade increases by 2".


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## mesenteria (Oct 29, 2015)

I'm pretty sure all of us who have built at least one solitary layout have had some surprises. I am on my fourth layout, just now laying some terrain, but my tracks are proven, wired, and I'm able to play with some trains.

I knew that my Trix GG1 with its pantograph was likely the highest item on my roster, even higher than that one True Line CPR Caboose's stack and the apex of the nested boom on my Athearn CPR wrecker crane. I was very careful to measure, then remeasure, and then record on my master drawing, the minimum height I would need for clearances at tunnel portals and under bridges. That all worked out fine when I finally tested it.

Then came the day I wanted to run the GG1 when I was putting all my roster of locomotives over the double main to find derailment spots along curves....which is always where they happen, especially at speed. Imagine my surprise when, after having completed almost the entire double main folded loop, and with my back turned, I heard a thump. I turned and found my GGI lodged firmly at the entrance to my double-track Pennsylvania-Petit Pratt truss bridge going nowhere fast and grinding away on the rails. I hit emergency stop, and stared in disbelief. The pantograph did not separate from the top of the GG1, thanks be to God. Nor was the hand-built wooden bridge damaged. But I clearly goofed. I had all my clearances correct EXCEPT for this one I never bothered to check. I had assumed everything would run easily on this through-truss bridge. The height clearance was fine; it was the four angled braces at the upper corners that just caught the edge of the pantograph.


My long point? That very excellent NMRA gauge is an extremely useful tool that every person in the hobby should acquire. But it isn't designed to show the clearance under structures for a raised pantograph. Not in every conceivable instance it isn't.


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## SantaFeJim (Sep 8, 2015)

gregc said:


>


Nice pic of the gauge tool. But he needs some dimensions for "H" or "C" "D" and "E".


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## gregc (Apr 25, 2015)

SantaFeJim said:


> Nice pic of the gauge tool. But he needs some dimensions for "C" "D" and "E".


here a link to the NMRA S-7 Clearances, but why not purchase the gauge? I use mine often enough


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## CTValleyRR (Jul 26, 2014)

gregc said:


> here a link to the NMRA S-7 Clearances, but why not purchase the gauge? I use mine often enough


Definitely. It's something no model railroader should be without.


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## ajay (Jan 24, 2014)

Thanks, everyone. Yes according to my SCARM model there's just enough track length to keep grades below about 2% for a 4" separation, but I wasn't sure how much clearance to allow for the bridge itself or if there was some standard? Here's a plan of the peninsula, on a shelf layout, where this will take place (the circular return loop is 28" radius -- so much for a 30" minimum that I wanted, lol). I'm worried about grade + curve.


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## gregc (Apr 25, 2015)

you should also consider the grade due to curve. There's obviously more friction when pulling a train around a curve. 

The rule of thumb for the "effective grade" on level ground is the radius / 32. So the effective grade of a 32" grade is 1% and 1.1% for a 28 radius curve. If you have a 28" curve with a 2% grade, the effective grade is 3.1%.

looks like i'm not the only one who has been disappointed attempting a 270 deg curve and crossing over the same track.


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## Shdwdrgn (Dec 23, 2014)

I think you meant 32 / radius? Otherwise I like your formula better because it would mean that a smaller radius has *less* friction.


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## gregc (Apr 25, 2015)

Shdwdrgn said:


> I think you meant 32 / radius? Otherwise I like your formula better because it would mean that a smaller radius has *less* friction.


yes ... thanks for the correction -- radius / 32


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## Dennis461 (Jan 5, 2018)

More than one version of the NMRA track gauge depending on era modeled.
This link is to a PDF with dimensions to match the letters.
https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-7_2012.02.pdf

As for the clearance, why not have a bridge which opens via track sensors ?
Could make for some additional excitement


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## ajay (Jan 24, 2014)

gregc said:


> you should also consider the grade due to curve. There's obviously more friction when pulling a train around a curve...If you have a 28" curve with a 2% grade, the effective grade is 3.1%...looks like i'm not the only one who has been disappointed attempting a 270 deg curve and crossing over the same track.


Doh! I was hoping this would be easy  I'd hate to create effective grades around 3%, which are required according to your helpful table above. How big a problem is that for the average loco pulling 10 or 20 cars?

So maybe I should go back to an earlier, simpler plan with no crossover & a level return loop rather than a tilted return loop. It's just that a track-over-track feature would add a lot more visual appeal & it's the only possible place to put it. But I wouldn't want my locos struggling, either.


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## Shdwdrgn (Dec 23, 2014)

I've been testing an 0-4-0 and an 0-6-0 on an 18.5" radius loop by setting one end of the plywood on a 2x4 (1.5" rise over 48 inches gives a grade of about 3.13%). I am pulling four cars fitted with all-metal Kadee trucks and wheels. With a little run they can *just* make the grade. If you stop and try to restart the locos while facing uphill they just spin their wheels. Now mind you, the electric motors have plenty of power to actually pull the load up the small hill, the problem is traction. The 0-6-0 does a little better than the 0-4-0, so more wheels provide more traction. Also note that neither of my locos have the little rubber bands on the drivers. Adding weight on top of the locos also makes a huge difference, but how do you get that weight hidden inside?

So yes, if you can get good traction then the loco should easily handle a train of 10 cars or more going up that grade, but traction is always the problem and not easily solved. In my own case, consisting a pair of 2-8-0's to haul a line of coal cars through a mountain setting will look perfectly natural and the additional driver wheels on the track should make all the difference. For your own setup...? That's something you'll have to decide how to handle.


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## mesenteria (Oct 29, 2015)

A previous poster pointed to a possible solution for you. If you have a really strong hankering for an overpass, then you can split the height of the vertical separation required by making the nether tracks go into a dip with its nadir right at the structure. The disadvantage of this is that it is more involved; you must craft not two grades that work, but four. Even so, you get your cake and you get to eat it later.


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## CTValleyRR (Jul 26, 2014)

Dennis461 said:


> More than one version of the NMRA track gauge depending on era modeled.
> This link is to a PDF with dimensions to match the letters.
> https://www.nmra.org/sites/default/files/standards/sandrp/pdf/s-7_2012.02.pdf
> 
> ...


There aren't multiple versions of the track gauge for different eras. Only one per scale.

You are correct, though that the HO standards gauge does not cover all contingencies in the NMRA Standard. The gauge more or less replicates the "Classic" era standards; if you have more modern equipment, you will need to check.

The standards gauge, though, has many more uses than defining clearances, and it is really these that make it an essential tool for any hobbyist.


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## gregc (Apr 25, 2015)

ajay said:


> How big a problem is that for the average loco pulling 10 or 20 cars?


what's an average loco? In my case, my 0-4-0 Mantua camelback with some traction wheels was ok, but my 2-8-0 Bachman couldn't pull more than a car or two. 

I could have tried adding some weight to the locomotive, but I assume there would have been some limit on the number of cars, hence why 2% effective grade is a common limit.


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## MtRR75 (Nov 27, 2013)

mesenteria said:


> A previous poster pointed to a possible solution for you. If you have a really strong hankering for an overpass, then you can split the height of the vertical separation required by making the nether tracks go into a dip with its nadir right at the structure. The disadvantage of this is that it is more involved; you must craft not two grades that work, but four. Even so, you get your cake and you get to eat it later.


That previous poster was me.

A long time ago, when the kids were into trains, we just used sectional track on an 4'x10' sheet of plywood mounted on sawhorses. Nothing permanent. We had a double oval that merged at one end and went over and under at the other end. The "under" track was level.

When we discovered that trains slipping too much while climbing on the "over" track, I shimmed up the end of the layout until the "over" track had about the same grade up as the "under" track did going down. Then we were able to run longer trains with less slippage. Quick fixes keep kids interested.

The original poster could use the same thing (board on sawhorses) to test how much his locos can pull on the grades that he wants to use.

But if you use curved track, things change. Curving track up an incline decreases the grade (because the track is going diagonally), but curved track gives you more rolling resistance than straight track at the same grade level. In my experience, the resistance increase effect on curves is greater than the decreased grade effect on curves.


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## ajay (Jan 24, 2014)

Thanks again for everyone's comments. I learned a lot, especially about effective vs. actual grades on curves. It also helped me focus my mind on this design issue that's been holding back progress.

I was able to tweak my track plan in Scarm to decrease the grade from 2% down to 1.7% through the 180-degree, 28-inch radius return loop--partly by increasing grades along less curved sections. Hence the worst-case effective grade will be about 2.8%. If my locos can't handle that, I guess I'll need new locos!

Assuming plenty of traction is available, is there still an increased risk of derailments on such a graded curve? Should I worry? Would it help?! 

I attached some images, but--sorry--I'm not sure how to insert them inside the text.

As for vertical separation: Silly me, I forgot to consult the master -- John Armstrong's _Track Planning for Realistic Operation_ (Kalmbach 1979), Table 5-10 on p.49 (attached). He shows a minimum HO separation as only 3", which is funny since that's the required NMRA clearance & allows zero thickness for the bridge deck. But then he says,"It gives the rock-bottom minimum grade separation which you can use if you are willing to go to the trouble of building an abnormally (in fact, almost dishonestly) thin-floored bridge for the upper track and pass up operating post-1960 freight equipment."​
So how thick is a bridge floor? Well duh, the answer was staring me in the face. One of the few finished structures on my layout is a plate girder bridge over a highway. The photo (attached) shows a ruler attached to one end, with the 1-inch mark aligned with the bottom of the bridge, camera level with rails. As you can see (arrow) the railhead is at just under 3/4" (5.4 scale feet) of thickness above the bridge's botttom, including cork roadbed underneath flex track. Added to the NMRA 3" vertical clearance, that makes 3.75 inches of grade separation necessary for this kind of overpass.

This finding allowed me to decrease the designed vertical separation from 4" to 3.8", which also eased the grades a bit.

If I had a simple 28-inch radius, 270-degree curve like in Armstrong's up-and-over diagram, its length would be 188 inches. My loop is 225 degrees (180+45) rather than 270, with an elongated loop of length 234 inches (per Scarm). The extra 46 inches helps relax the grades. The hand-drawn diagram, attached, shows elevation data normally invisible at this zoom level. Those are the numbers (in inches) I'll need to set the risers.

Btw, most of my shelf layout takes advantage of combining opposite grades. Maximum and minimum elevations occur at roughly the layout's midpoint, then the high track descends at 2% while the low track ascends at 2%, so they effectively converge at a 4% rate. The loop in question, however, is on a peninsula beyond this convergence, which you can see in the Scarm 3d rendering.

This is my last chance to change back to a level loop with a flat 45-degree crossing instead of an overpass. But I think this is going to work, by golly! Lol, any dissenters?


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## Shdwdrgn (Dec 23, 2014)

Might I point out that you don't use cork roadbed over the bridge? The track is normally attached directly to the deck of the bridge, so the way you have it mounted in that photo above makes it look like the majority of the bridge was filled in with dirt before the track was laid. Used correctly, I think that bridge should be almost half an inch higher than it is, and for your tight grade that half an inch can go a very long ways.


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## MichaelE (Mar 7, 2018)

Try not to overthink this. It's not rocket science. I see this as just one of many obstacles that keep railroad layouts on paper without a hammer or saw ever being used to actually build one.

Planning and getting it right on paper is one thing, but obsessing over minute details will keep your layout from ever being built. You're not laying track for Canadian National.


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## ajay (Jan 24, 2014)

Thanks, Shdwdrgn. Since I'm using spline sub-roadbed, in order to make a bridge I first have to cut away, using a Sawz-all, a section of spline sandwich beneath the cork & track, which have already been glued down. The cork helps support the bridge span of otherwise bare track suspended in midair at this point, while the bridge gets installed from below. At least that's how I've been doing it (3 bridges so far). At the rather glacial pace I work at, months or years might elapse between laying the splines & fitting the bridge.

I guess next time I'll remove the cork, too, before fitting the bridge to the track. Thanks for your sharp eye! That's the kind of rookie mistake I'll probably keep making, since I'm doing this project alone & never undertook a layout this complex & just figuring it out as I go, using old books and new websites. Thank goodness for online communities of experienced modelers!


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## Shdwdrgn (Dec 23, 2014)

I'd agree with Michael's statement. My own layout has been in planning for the last four years while I wrap up other projects to make room, and during that time I have fretted greatly over my mountain grades (which started out at nearly 4% not including the drag from the winding mountain path). During that time I have come to realize that despite all the theories and formulas, nobody can tell me exactly what will work. So when I'm ready to start laying track I will keep my framework completely open and run the actual locomotives with various loads, and I will see exactly what works, what requires consisting, and what absolutely must be lowered to allow my trains to operate reliably. You already know that a figure-8 can be done in a fairly small space, and you are working with a larger area so you have room to play. Build it, try it, if the trains can't handle the grade then lower it until they can... and once you have that all working THEN you can add the bridges and other things with the consideration of how much clearance you still have available. There's a way around almost every problem, and there's a lot of experience available here on the forum to help provide solutions if you see any problems.


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## trenes115 (Dec 9, 2011)

There is an NMRA modern era clearance gauge. The plastic gauge works in conjunction with the regular metal NMRA standards gauge. https://www.nmrastores.com/Public_Store/product_info.php?cPath=27&products_id=100&osCsid=ojs1j1d104d9np9rephb16n943


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## CTValleyRR (Jul 26, 2014)

trenes115 said:


> There is an NMRA modern era clearance gauge. The plastic gauge works in conjunction with the regular metal NMRA standards gauge. https://www.nmrastores.com/Public_Store/product_info.php?cPath=27&products_id=100&osCsid=ojs1j1d104d9np9rephb16n943


A useful addition to their products, and I'm glad they've done that. But the standards gauge remains unchanged -- one version only.


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## CTValleyRR (Jul 26, 2014)

MichaelE said:


> Try not to overthink this. It's not rocket science. I see this as just one of many obstacles that keep railroad layouts on paper without a hammer or saw ever being used to actually build one.
> 
> Planning and getting it right on paper is one thing, but obsessing over minute details will keep your layout from ever being built. You're not laying track for Canadian National.


That is the best advice you will ever get, right there. Whether you call it "paralysis by analysis" or something else, it is something you want to avoid.

Don't just charge blindly into things -- have a good idea where you're going before you start -- but you don't need to nail every little detail dead on, or mitigate every possible risk, before you start building. Things probably won't go together exactly as you planned anyway.


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## ajay (Jan 24, 2014)

> "paralysis by analysis"


I like that . Yes, that is my tendency  though once I settle on a track plan I will resume building in earnest.

Years ago my wife & I were fortunate enough to have a new house constructed. Once the plans were drawn up there came a brief moment, right before they staked out the foundation, when we could (in effect) pick up the whole house and move it or turn it, a little this way or that, for best placement at the site. After that it's of course impossible & you live with it.

This is such a moment with my layout. The track plan is easy to modify now, then unthinkable once all the risers, brackets, & diagonal bracing are installed & splines glued up & screwed down--impossible to change without massive rebuilding & life's too short . Ounce of prevention & all that. I appreciate everyone's input!


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## Geno the Viking (Feb 29, 2012)

I've read this entire thread and thanks for all the info, but as a newby building my cross over right now I'm still going to get your opinion. Currently I have calculated a 2 7/8 top of rail to bottom of my upper structure maximum height clearance. My grade is 2% with 30" radius curves. I'm modeling HO 1967 era and older. Does anyone think I will encounter any problems. It's going to be a real challenge to get more clearance. Thanks.


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## CTValleyRR (Jul 26, 2014)

Well, the standards and clearances are established for a reason: to make sure that things run correctly. 

That said, there is no substitute for empirical evidence -- mock it up, and before you make anything permanent, test it.

The only problem then will be that even though all your current equipment works, future acquisitions might not.

Ultimately, it's your call. Your layout, your rules.


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