Clearance under a bridge--standards versus practical?

[Zox]

I'm building a highway overpass on my newest module, and I want to double-check the overhead clearance (from railhead to bottom-of-bridge).

NMRA standards say that the minimum clearance for "Modern Dimensions"  is 23 feet, or 1²³/₃₂" in N scale.

Right now my abutments put the lowest clearance at 25 feet 5 inches, or 1²⁹_/32" in N scale, ³_/16" over the alleged minimum.

Does anyone know of any N-scale rolling stock that will have trouble fitting under this bridge? I know sometimes N-scale gear is taller than prototype, but I don't know if there's already a "fudge factor" built into the NMRA numbers.

[daniel_leavitt2000]

Possibly with the Walthers well car. Those stand tall if highcube containers are used.

[Robbman]

If the NMRA clearance is 23', then there's some fudge factor built in it.  Plate F+ (doublestacks) is 20' 2", most roads use 20' 5" to 20' 8" as the max clearance for Plate F+ cars

[Zox]

Possibly with the Walthers well car. Those stand tall if highcube containers are used.

Thanks. Any chance of someone putting calipers to this combination and finding out how high it actually is, railhead to top of load? I don't have a sample to test, so the information would be appreciated.

If the NMRA clearance is 23', then there's some fudge factor built in it.  Plate F+ (doublestacks) is 20' 2", most roads use 20' 5" to 20' 8" as the max clearance for Plate F+ cars

After a bit more research, if this PDF document from Transport Canada is to be believed, then the NMRA simply converted the prototype dimension to scale, without making any allowances for modeling error.

As you say, the height of the "loading gauge" (the size of the cars) tops out at 20'2". The height of the "structure gauge" (the size of the minimum clear space around the tracks), however, is 23', just like the "1:1" NMRA spec.

Yet another "NMRA standard" that's really someone else's work with a  label slapped on it...

[GaryHinshaw]

Thanks. Any chance of someone putting calipers to this combination and finding out how high it actually is, railhead to top of load? I don't have a sample to test, so the information would be appreciated.

I can do this later today and post the results.  I agree that this is probably the highest combination you are likely to find.

-Gary

[mmyers]

Rob,
 I use 2 inches for NTRAK modules. A little high but very safe. In NTRAK at least. the masses can remove the bridge to maintain traffic on the three "public" tracks.

Martin Myers

[GaryHinshaw]

Lord Zox,

I checked the stock Walthers 5-unit Thrall well car set with 2 Deluxe hi-cube containers and the set tops out at 1^20.2/₃₂" (21.75 scale ft) above the railhead.  For comparison, a more prototypical stack set (the Kato Maxi-IV with Kato containers) tops out at 1^17.8/₃₂" (20.75 scft - still higher than Plate F+).  So you should be completely fine with your clearance.

HTH,
Gary

[Zox]

Thanks, Gary--that's what I needed to know. I can now forge ahead in confidence that, while I might screw up everything else, at least I won't be blocking the mainline.

[Robbman]

After a bit more research, if this PDF document from Transport Canada is to be believed, then the NMRA simply converted the prototype dimension to scale, without making any allowances for modeling error.

As you say, the height of the "loading gauge" (the size of the cars) tops out at 20'2". The height of the "structure gauge" (the size of the minimum clear space around the tracks), however, is 23', just like the "1:1" NMRA spec.

Yet another "NMRA standard" that's really someone else's work with a  label slapped on it...

That document outlines the practice for NEW structures built in Canada since 1992... but I can tell you with certainty there are very few structures that meet that clearance.  Norfok Southerns' Heartland Corridor project that was completed recently used a maximum height of 21'.  NS' own standard is 20' 5"...

[Zox]

And it's already pretty obvious that this scene is not going to match the prototype as closely as my other bridge:





Oh well. Good thing I got my modeler's license renewed...