Wouldn’t that be for clearance, space to the next track to avoid passing trains colliding ?

I was using Atlas sectional track and stuff that comes in those cheap sets ( In other words garbage track – I use only Nickel silver and Flex track for most of my layout – except when I needed a short piece ). I saw something in an advertisement and needed to find out what the grade was after one installs the seven risers
( after creating that full circle, and counting the number of sections used the grade came out to over 4% — forget that noise). That is when I found out that if I go outside rail to Outside rail, that diameter was 34 inches. So how in the world these companies can hire kindergarten dropouts to put together track plans using what they call 18″ radius track is beyond me. I do have 22 and 24″ radius sectioned track ( from Atlas ), which when I find a couple of minutes, will assemble in a full circle and measure those to see where they come out — 44 or 48 diameter.

I’m at a loss to explain how you can assemble “18 inch radius curves” and end up with a radius of 17 inches to the outer rail!!! What make of track are you using? Marklin has curves of 16 3/4 inch radius which would probably give you 17 inches to the outer rail.

But as far as your planning is concerned, you are going about it the correct way. Set something up and measure it. And then use the same make on all of your layout. I used trackplanning software, which said it had a track library for the track I was using, but when I got to actually laying it out, I found that the required clearance between tracks in the yard meant that my actual yard was one track less than my plan!