Layouts and layout building

 Radius is measured --to the inside track--to the outside track --center line ?

I have always assumed that it was the center line.

 Radius is measured from the center point of the curve to the centerline of the track. HO track is 11/4 in wide, with standard cork roadbed just under 2in wide.

So a 18in radius curve needs a width of about 38in for a full circle. 

Checkout Beginners Page Consist from NMRA. 

BTW the NTRAK standard is  24in radius for mainline, 18 for branch.

 Many thanks

Since I have a 4 X 9.5 , I am going with a 22 outside oval and 18 inside oval with a cross over at the bottom

I will be close to the edges with the 22 but with a wall on the one side of the long dimension and a 2 'high plexiglass  on the exposed side I should be safe from catastrophic  derailments (kind that land on the floor0

Will run Mountains, Mikados and Consolidations on the 22 and Consolidations, Moguls and Americans on the 18 with ocassional crossovers --no cars longer than 40 ft

UncBob:

Will run Mountains, Mikados and Consolidations on the 22 and Consolidations, Moguls and Americans on the 18 with ocassional crossovers --no cars longer than 40 ft

Probably but chances are not well, especially the mountain.  Mountains usually like something in the 24-30 in radius range and look funky on small radius curves, below 30".

 Been running IHC moutains on 22 and no problems  and they look great pulling 16 coal cars

Thing is I have had just a plain painted green homosote over plywood with one 22 oval for years and have decided to  expand and do scenery etc and get a little more serious with 2 train capability

Been using Bachman snap track and am converting over to Atlas 100 

 22" may not be optimal, but it will accomodate 95% of everything on the market.

 Here is a pic I just snapped of the Mountain --I am testing my new track before tacking permanently and balasting

Radius measurements can be from any point on the track to the same point on the opposite side. Just watch you don't try going from outside to outside rails. It would be measured from left-most rail to left-most rail on the opposite side, etc.

edit- I would also clarify a little, if you are driving a nail in the center of the circle and drawing the circle, it would be the centerline.

tinman1:

Radius measurements can be from any point on the track to the same point on the opposite side. Just watch you don't try going from outside to outside rails. It would be measured from left-most rail to left-most rail on the opposite side, etc.

 Aren't we speaking of an arc (a segment of a circle's circumference) when we discuss radius issues?  It's a point of reference and what we actually incorporate into our layout.

My error. Radius is from the reference point in the center, more useful than diameter, especially with less than 180 arcs. Diameter is great just to see if the track will actually fit , provided that one remembers to add the width of one track to it. (ie; a 18"r will not fit on a 36" table. In HO it needs 38"min, from edge to edge).