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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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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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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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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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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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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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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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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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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
Code:
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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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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Discussion Starter #17
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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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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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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