Model Railroading > Over and Under

I am building a HO layout. When one track crosses over another track, what is the minimum distance
from the top of the lower rail to the bottom of the bridge?
 

Ideally you'd want about 22 scale feet, three inches in HO scale.

What should be the maximum % grade I should use to get to the  3 inch height?

I think AREMA calls for 23 ft 4 inches.
If I recall the last overhead I had input on was at 24 feet.

The railroad that grants the easement is in the drivers seat.



Edited 1 time(s). Last edit at 04/14/24 15:02 by train1275.

They used to say 3 in. to 3-1/32,  but if you are modeling modern cars like auto-racks and double-stack containers you need 3-5/32 in. from top of rail to underside of overhead obstruction. Having said that, I'm modeling the steam, transition era and my tallest rolling stock, a Bachmann Hudson, may get by with  2-3/4 in. (I am going to test that before I lay the roadbed and risers.) 22 ft. in HO is about 3-1/32 in.

Victor Baird



Edited 4 time(s). Last edit at 04/14/24 15:20 by wabash2800.

Depends on what you are running and how long your trains are. The general rule of thumb is to not go over 2%.  But short trains and small layouts often use 3%.

Victor Baird

JunkTrain Wrote:
-------------------------------------------------------
> What should be the maximum % grade I should use to
> get to the  3 inch height?



Edited 1 time(s). Last edit at 04/14/24 15:24 by wabash2800.

My minimum clearances in HO scale are exactly 3" and everything I run - double stacks, autoracks, high/wide loads - clear with at least 1/8" to spare.

wabash2800 Wrote:
-------------------------------------------------------
> They used to say 3 in. to 3-1/32,  but if you are
> modeling modern cars like auto-racks and
> double-stack containers you need 3-5/32 in. from
> top of rail to underside of overhead obstruction.
> Having said that, I'm modeling the steam,
> transition era and my tallest rolling stock, a
> Bachmann Hudson, may get by with  2-3/4 in. (I am
> going to test that before I lay the roadbed and
> risers.) 22 ft. in HO is about 3-1/32 in.
>
> Victor Baird



Edited 1 time(s). Last edit at 04/14/24 16:41 by 4thDistrict.

JunkTrain Wrote:
-------------------------------------------------------
> What should be the maximum % grade I should use to
> get to the  3 inch height?

You need to know the length of the approach you want in order to calculate the grade needed. Be sure to add the distance from the bottom of the bridge to the top of the rail to the clearance you need,

 

While you do the math to figure out how much run you'll need to climb 3"+, don't forget to leave some transition room at the top and bottom for vertical curves.  I didn't do a good job handling mine for a very simple little climb up from the staging track (flat on the subroadbed) to the mainline (on Woodland Scenics trackbed) and had my 89' cars uncoupling left & right.  Thought that would be a great way to prevent runaway cars, but instead I ended up leaving half the train or more in staging.  :(  I'm still fixing spots from that little whoopsie.

Nathan

Generally 4" subroadbed to subroadbed provides enough clearance for just about anything plus the 3/4" plywood subroadbed. If you model a earlier era with no Hi-cube cars, then 3.5" should work. You can go even less if you measure the height of your tallest cars on track and roadbed material and adding the depth of the supporting bridge structure on the overpass.

Ted Curphey
Ontario, OR

JunkTrain Wrote:
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> get to the  3 inch height?

As mentioned three percent should be about as steep as you'd want to go. But it depends on what your modeling and how much space you have.

If your running more modern 60 to 90 foot equipment you may wish to stick with two percent or less.
Logging you could go over three percent.

Three percent means your track would go up or down three inches over one hundred inches of track length. For two percent it would be one hundred and fifty inches of track length to gain three inches. As Nathan pointed out, it's a good idea to allow room for a smooth transition on each end.

funnelfan Wrote:
-------------------------------------------------------
> ... If you model a earlier
> era with no Hi-cube cars, then 3.5" should work.
> You can go even less if you measure the height of
> your tallest cars on track and roadbed material
> and adding the depth of the supporting bridge
> structure on the overpass.

Just remember though that your friends won't be able to bring their modern, tall equipment over and run it on your layout.

I was going by what the NMRA standards are. But I seem to recall a Walthers truss bridge that had to be heightened above 3 inches later in production as it would not clear double stacks?

Victor Baird

4thDistrict Wrote:
-------------------------------------------------------
> My minimum clearances in HO scale are exactly 3"
> and everything I run - double stacks, autoracks,
> high/wide loads - clear with at least 1/8" to
> spare.
 



Edited 1 time(s). Last edit at 04/16/24 17:18 by wabash2800.