2: AAR plate C clearances for interchange, from the Official
Railway Equipment Register.
The time-honored NMRA gauge, in HO, measures 21' 6” high
from the railhead and 14' 6” wide, tapering to 8' wide at the
top. It looks a lot like the Association of American Railroads
(AAR) Plate C standard, adopted in 1968 and revised several
times since. Plate C allows 15' 6” of height and 10' 8” of width.
So, what's the big deal? After all, the NMRA gauge allows 6'
more height, and 3' 10” more width.
Trouble is, modern freight cars are long, and they don't always
taper at the top. In 1991, the AAR published Plate F standards,
allowing a maximum car height of 20' 2” above the top of the
2
3: AAR plate F clearances for interchange, from the Official
Railway Equipment Register.
rail. Unfortunately, the same standard allows a maximum width
of 8' 6 3/8” at maximum height. Our freight car was rolling
through a curve, increasing the car's overhang on the inside of
the curve. AAR standard S-2041-94 sets the maximum width
of cars with various track centers. So a long car with its truck
centers spaced at 80' can be only 8' 3” wide to meet clearances
because so much of it will overhang the inside of a curve. That
NMRA gauge allowed only 8' of width at maximum height.
Bang, scrape, and maybe a car derailed. We are still within
clearances, so what happened?
We're not done. The AAR maximum widths are based on a
curve radius of 441' 8 3/4”. That's about 60” when scaled
3
AAR rail clearances - 2
MRH-Jun 2013
