As the topic states, where do you measure the center for radius and diameter from on curved rail? The inside rail, the outside rail or the center of the rails?
Thanks
Terry
My understanding is that it is the center, between the rails. Of course, I’ve never really checked to be certain.
-Kevin.
That’s what I thought too but honestly never have checked.
Terry
Terry all of my curves are measured from the center line in between the 2 rails.
Terry, the radius and diameter of a curve are measured from the center line of the track. Just like we do in road construction, the radius is measured along the centerline. I use flextrack on my layout. I decide on where the curve is to be located, determine where the beginning and ending points will be, then do an arc to locate the center of the curve and paint the curve centerline and lay the track on top of the line. Piece of cake!
Ron
Technically it should be measured on the center-line of the track (between the rails). Not everyone does it that way, but they are supposed to.
When I laid out my curves I measured to the inside rail, since I was bending my own. That way I was certain my 10 foot diameter curves would accommodate equipment the “required” 10 foot diameter curves. And then I got to the Fairhaven loop. I had to cheat that curve in to 9.5 feet, to stay clear of my neighbors property.
Diameter, I have aways measured from center of the rails to the center of rails.
Radius, half of the diameter.
Don
Chuck Inlow said:
Terry all of my curves are measured from the center line in between the 2 rails.
Practically speaking, the margin of error if you measure from the inside or outside is not worth worrying about (unless you are a fan of LGB R1 2’ radius curves!) On a 6’ radius curve, that 7/8" is about 1% error.
This is another one of those LS peculiarities. Some mfg(s) had to muddy the waters by stating diameter, but then someone went one better and started stating the diameter to the outside of the ties. 
BTW LS has an exclusive on this nonsense, everyone else states a radius and that’s it.
Except for some NA RR engineers who designate curves in degrees, which is also a lot of fun. 
It’s the “we don’t need no stinking standards” syndrome!
Thanks for the input.
Terry
And sometimes they say “3 ft” when it’s really 90cm. Turns out, track isn’t a very precise thing.
The LGB geometry was in mm, all foot notations are aproximations.
When you think about it:
If you measure the track “left rail to right rail,” or “inside rail to outside rail,” you’ll get the same measurement as center to center.
Echoing the comment from Tom about preciseness I would add that you only have to visit many 1:1 railroads to see how imprecise track can be.
As mentioned as to the way 1 to 1 RR figure curves they use degrees for determining curves. Yes you can use center of track to make your determination but as mention is what I do most is use the outer rail for measurements. No big deal. Later RJD