I cannot seem to get a definitive answer on this.  I am currently designing a shelf mounted track for my son's room and this is my first foray into model trains.  I've learned that many sets come with O-36 FasTrack.  I was told that this would create a 90 degree curve with a radius of 18" and an overall diameter of 36".  However, how is this measured?  Is it to the center of the track (middle rail)?  the outside rail?  inside rail?  or the inside/outside of the track bed (plastic)?  Several website say outside rail and several say middle rail.  I would assume it would be to the middle rail (assuming it is centered in the overall piece of track).  In designing the track layout, I'd like to keep the train track the same distance from the edge of the shelf all the way around the room.  Assuming wrong would have several implications:  1.  In the corners of the room, the train track would either move closer or further from the edge of the shelf than the rest of the room.  2.  It could cause the train to hit the wall as it rounds a corner either causing damage to the wall, the train, or both. 

As a follow up to this question.  Using an O-gauge train set (example, Polar express set, SKU: 6-30218), what is the width of the train?  Also, when the train enters a corner, what kind of clearance does it need so it doesn't hit the wall?  Will the train go out past the plastic rail bed as it goes around the corner?

Thanks for the help in advance!

Thanks!  That helps a lot!  However that brought up another question. What is the overall width of the Lionel FasTrack?  I've read that the width of the track bed is 3.5" inches.  Therefore, there is 1.125" then a rail to rail width (inside to outside rail) of 1.25" and then another 1.125".   So in the above example, I'd have to plan about an overall 5.5" width, 3.5 for the track and 1" on each side for cushion.

Thanks everyone for the quick responses.

Maybe I'm over planning it but when I order the set, I want to order all the additional track I need.  I would like it to look uniform (same distance from the edge of the shelf) going all the way around the wall so measuring out the widths of the track and cushion, I think, would be important. 

Has anyone made a shelf layout where you mount the shelf about 10" or so from the ceiling?  Is that a decent distance from the ceiling?  Also, how far from the edge of the shelf do you mount the track?  0.5"? 1.0"? 

My current design has a 6.5" shelf (and in order from the wall towards the center of the room) where:
0.5" is a LED rope light
1.5" cushion
3.5" Fastrack
1.0" cushion

Or would it make more sense to have the train closer to the edge of the shelf, say 0.5" cushion (replacing the last 1.0" cushion, or the edge closer to the center of the room)?

I'm going to put together a YouTube video of the install and complete project and will post it once I'm done.  (It may be in the new year!)

Many thanks!