# Overpass Height.



## richs75 (Jan 21, 2016)

First, let me introduce my self. 
I am getting back into HO railroading after being out of it for over 30 years.
Have modeled in HO when I was young, then in N scale in my early years of marriage when my boys were small. Now, my boys have small boys and figured it would be the perfect time to get them interested in trains.

Going to start an HO layout and can't seem to find the answers to a few questions about layout design using the search feature.

#1 What is the minimum height from track to track for an overpass, where one track is crossing over an other track?

#2 What are the radius (radii ?) of a #4 and a #6 turnout?

#3 What is a good grade for track elevation that works good today's engines and a small train, say a dozen cars or less?

Thanks for any other help and I am sure I will be posting a lot of other questions in the near future.


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## tkruger (Jan 18, 2009)

#1, What I did was to take the tallest piece I had and add a bit to the top. Remember to include the rail height to that also. This is not an exact practice but guarantees everything you have will fit. Down side as I found out in my prior layout is that it does not account for future equipment.

#3, Keep everything under a 2% grade. Also understand that turns increase the pulling effort required. For this reason a curve on a 1% grade is actually the equivalent to a steeper grade.


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## D&J Railroad (Oct 4, 2013)

I don't think you want to measure rail head to rail head for clearance. You need something to support the track on the overhead part. As tk said, measure for tallest item you will be running, or buy a NMRA gauge tool. The modern era equipment height would be 3 5/32" minimum. If your track is not level for at least a car length of your longest rolling stock, you will have to increase the height clearance depending on close to the underpass that grade begins.


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## mesenteria (Oct 29, 2015)

richs75 said:


> First, let me introduce my self.
> I am getting back into HO railroading after being out of it for over 30 years.
> Have modeled in HO when I was young, then in N scale in my early years of marriage when my boys were small. Now, my boys have small boys and figured it would be the perfect time to get them interested in trains.
> 
> ...


For overhead clearances, you must measure from the rail head below to the closest obstacles above it. This might include the structure for a bridge or a tunnel portal. So, in your calculations, you must consider the height of your tallest item passing under the overpass. Is that the stack on a caboose, the boom on a wrecker crane...maybe the height of the top container on a double stack if in modern times? Generally, 3" from the top of the bottom rail is good, but only your intentions for rolling stock will be the real determinant.

Add your roadbed, ties and rails to that overhead structure and you get the height you must reach on your grade. Many of us accept grades around 3%, especially on smaller layouts. I won't do that any more...it's too hard and a bit unrealistic if you want to run realistic lengths of passenger trains without doubling. I have found that what the Cumberland & Wheeling took as their standard grade, and what Congress then took as their maximum grade for railroads wanting federal assistance, is a reasonable standard, and that is 2.2%.

The substitution radius for standard pattern turnouts (as opposed to the tight curved diverging route turnouts in the Code 100 rails) is only that...a substitution along a curve. You'll have to find the charts in the NMRA site for the style you are using, and of course the frog number, but a standard #6 turnout with the straight diverging route after the frog, has a substitution radius of about 46" if I remember correctly. A number four standard pattern would be about 26" or thereabouts.


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## DonR (Oct 18, 2012)

The correct answer to all of your questions is in the
NMRA standards.

http://nmra.org/index-nmra-standards-and-recommended-practices

Don


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## richs75 (Jan 21, 2016)

tkruger said:


> #1, What I did was to take the tallest piece I had and add a bit to the top. Remember to include the rail height to that also. This is not an exact practice but guarantees everything you have will fit. Down side as I found out in my prior layout is that it does not account for future equipment.
> 
> #3, Keep everything under a 2% grade. Also understand that turns increase the pulling effort required. For this reason a curve on a 1% grade is actually the equivalent to a steeper grade.


2% grade would work well for my proposed layout. Thanks.




D&J Railroad said:


> I don't think you want to measure rail head to rail head for clearance. You need something to support the track on the overhead part. As tk said, measure for tallest item you will be running, or buy a NMRA gauge tool. The modern era equipment height would be 3 5/32" minimum. If your track is not level for at least a car length of your longest rolling stock, you will have to increase the height clearance depending on close to the underpass that grade begins.


I should have explained better. I had planned on adding the height of the upper track and trestle structure into my clearance height. The 3 5/32 is what i needed. That would make about a 4" rail to rail height that I am looking at. More on that later.





DonR said:


> The correct answer to all of your questions is in the
> NMRA standards.
> 
> http://nmra.org/index-nmra-standards-and-recommended-practices
> ...


Thanks Don. That's exactly what I needed. Great info there.


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## richs75 (Jan 21, 2016)

Alright, so a refresher on grade for my benefit.

Grade is relative as 1 foot per 100 feet being a 1% grade, right?

So, if I need 4 inches from lower rail to upper rail, that would be 400 inches to 1% grade. So a 2% grade would be half of that, correct. 200 inches or about 16 ft. of track. 

Am I on the right track, pardon the pun?


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## RH1 (Jan 4, 2016)

richs75 said:


> Alright, so a refresher on grade for my benefit.
> 
> Grade is relative as 1 foot per 100 feet being a 1% grade, right?
> 
> ...


Yup. On the right track. Although you do want to "ease" in and out of the grade, so add a bit for each end.

One of the grades on my layout is closer to 3%, but with the equipment I'm running it works fine. I just didn't have room to make it any longer!


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## traction fan (Oct 5, 2014)

*Grade*



richs75 said:


> Alright, so a refresher on grade for my benefit.
> 
> Grade is relative as 1 foot per 100 feet being a 1% grade, right?
> 
> ...


richs75;

Yes, you math for determining percent of grade is correct. You may want to add a bit(about 2-3 feet) beyond your 200 inches for easements at both the bottom and top of your grade. 
What's an easement? It's a short section of a milder grade used to ease the transition from flat running to climbing a grade and then back to flat running. With only a 2% grade, the easements would be 1% or less. You could probably get by without them on that mild a grade. Many modelers use grades of 4% or more. Here the easement becomes more important since the transition is more severe. However, it's always good to include easements whenever you can. It makes trains handle the grade more easily.

Traction Fan


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## richs75 (Jan 21, 2016)

Thanks. That shouldn't be a problem. Thanks for the advice and help.


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## DonR (Oct 18, 2012)

The easement is very necessary at both top and bottom of the grade.
At top if not correct the front wheels of loco will leave the rails and
couplers will dig in at the bottom.

To create it, fasten down a piece of flex track about 3 inches back from where
your grade starts then lay the flex on the grade supports. 
The flex will create it's own easement. Do the reverse of that at the top
starting back on your grade before it tops.

Note: Make certain the flex joint is well back from where it is fastened
down to avoid vertical kinks.

Don


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## CTValleyRR (Jul 26, 2014)

And so far, no one has addressed the turnout question. You're thinking of the train set type of turnouts, where the diverging leg forms part of a circle. Real turnouts don't do that. The diverging leg comes off in a straight line.

The measurement (turnout number) is an indirect measure of the angle of departure, expressed as the number of units of run for each unit of divergence. In English, the diverging leg of a #6 turnout will have 1" of horizontal separation (measured from the centerlines) for every 6" you travel from the frog.

Welcome back, BTW.


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## richs75 (Jan 21, 2016)

CTValleyRR said:


> And so far, no one has addressed the turnout question. You're thinking of the train set type of turnouts, where the diverging leg forms part of a circle. Real turnouts don't do that. The diverging leg comes off in a straight line.
> 
> The measurement (turnout number) is an indirect measure of the angle of departure, expressed as the number of units of run for each unit of divergence. In English, the diverging leg of a #6 turnout will have 1" of horizontal separation (measured from the centerlines) for every 6" you travel from the frog.
> 
> Welcome back, BTW.


Let me understand this better. My plan was to use #8 turnouts on mainline where ever possible. #6 everywhere else. Trying not to use any #4 if I can get away with it.

My dad worked for the CNW Railroad for over 20 years (surveyor and Roadmaster for most of them). I remember him always telling me how the curve of a track starts out less gradual then the rest of the curve. Kind of thought maybe the same holds true for turnouts. I plan on using the flex track this way, starting the turnouts gradual and then a little tighter as the tracks lead away from each other.

For example, my mainline will have nothing less than a 24" radius, with #8 turnouts off them. After getting into side tracks, I plan to use #6 turnouts. Just didn't want to have too sharp a turn off to the sidelines that with probably be no tighter than a 20-21" radius. 

So I kind of thought that the #6 would be less sharp than a 21" radius?

Make sense?


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## mesenteria (Oct 29, 2015)

What he was describing is called a cubic spiral, or commonly, an 'eased curve'. It doesn't happen in turnouts to the same degree as out on the rails, but it does happen on railroads the same way cars perform their turns. It goes like this: first the steering wheel is centered and both front tires run parallel to the main axis of the vehicle's chassis. Then, as the driver begins to spin the steering wheel, the tires turn more and more, always reducing the radius of the curved path the driver intends. However, the car is also traveling 'forward', or along the tangent at each point of the curve where a radius can be drawn to the centre-point for the entire curve. While the driver is turning the wheel, the radii, from tangent to tangent point, become slightly shorter until they near the apex of the curve, at which the radii are constant, or essentially so. Then the process is reversed as the car needs to straighten out again to move along the next street or into a parking space.

In order to control_ lurch_, especially for fragile goods and passengers, the prototype rails had two qualities along the curves: superelevation and easements. The eased curves prevented the sudden and constant acceleration forces outward caused by perfectly circular curves, such as those found in sectional track, and the superelevation displaced the center of gravity of the rolling stock in towards the center of the curve to avoid tipping. Both characteristics made passengers safer when they were on their feet and helped pendulated chandeliers, for example, and soup in pots in the diner galleys, from sweeping outward and causing injury.

I use past tense, but the proto rails still use eased and superelevated curves.

To address your last statement, a #6 turnout's curved points will probably have a curvature mid-rail of about 38". Therefore, no, they are not sharper curves than the 21" radius.

A #6 turnout, to address your last statement that also ends with a question mark, and as I explained earlier, has a very wide substitution radius. The radius of the curved points rails, by themselves, is probably in the 35" range, so no, not sharper than the 21" radius you mentioned.

Many of us try to emulate both qualities, often overdoing them and just as often misunderstanding them. Superelevation on the rail rails rarely exceeded 6", and models with that much often look overcooked.


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## mesenteria (Oct 29, 2015)

For some reason the edit feature won't work for me. A #6 turnout standard pattern for N. America will have curved points rails in the 38" radius range, maybe a bit more. So, they are not sharper than 21" radius, they are much broader, nearly twice as much.


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## richs75 (Jan 21, 2016)

mesenteria said:


> What he was describing is called a cubic spiral, or commonly, an 'eased curve'. It doesn't happen in turnouts to the same degree as out on the rails, but it does happen on railroads the same way cars perform their turns. It goes like this: first the steering wheel is centered and both front tires run parallel to the main axis of the vehicle's chassis. Then, as the driver begins to spin the steering wheel, the tires turn more and more, always reducing the radius of the curved path the driver intends. However, the car is also traveling 'forward', or along the tangent at each point of the curve where a radius can be drawn to the centre-point for the entire curve. While the driver is turning the wheel, the radii, from tangent to tangent point, become slightly shorter until they near the apex of the curve, at which the radii are constant, or essentially so. Then the process is reversed as the car needs to straighten out again to move along the next street or into a parking space.
> 
> In order to control_ lurch_, especially for fragile goods and passengers, the prototype rails had two qualities along the curves: superelevation and easements. The eased curves prevented the sudden and constant acceleration forces outward caused by perfectly circular curves, such as those found in sectional track, and the superelevation displaced the center of gravity of the rolling stock in towards the center of the curve to avoid tipping. Both characteristics made passengers safer when they were on their feet and helped pendulated chandeliers, for example, and soup in pots in the diner galleys, from sweeping outward and causing injury.
> 
> ...


 Very good. A much better way of saying what I was trying to say. And answered my questions too.

So the eased curve part I can duplicate in model railroading.
Is the Super Elevated part something that is done in modeling too, or is it even needed?


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## mesenteria (Oct 29, 2015)

It isn't needed, but looks good and realistic, particularly in photos.


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## richs75 (Jan 21, 2016)

WOW, Love the second photo. Not a straight section of track there and that is the look I will be going for.


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