A search of the FRA website shows that there is no specific clearance required between adjacent tracks. In modelling a triple track mainline, I was wondering what center to center spacing to use on 18" radius curves. I have generally used 2" as a rough gauge however I was wondering if anybody has experience with this and care to weigh in? My main concern is maintaining adequate clearance on 80' passenger cars.
For curves, well, either do test runs with your own equipment or just run with something big and accomodating (say, 2.5" in HO or 1.5" in N). For tangent? I can give you two data points. (i) Awhile back I measured the centerline track spacings at about 15 crossing in the Houston Belt and Terminal and connecting railways (mostly UP). Generally speaking they were all on 13-foot centers, widening out on curves. (ii) I read SEPTA's entire procurement specs/rfp for the Silverliner V EMUs, and the design track spacing (for body envelope, roll, etc) was 12'9" in tangent. Personally 13' is cutting it a bit close on a model railroad. AAR plate C is 10'8" wide, and some cars are slightly off scale, so on 13' centers it's not unlikely that passing trains will strike each other. 15' is common on lines in the Northwest (BNSF/ex-NP) and that'll give you more reliable running.
After about 5 years running on my testbed layout I have settled on the following track spacing: 1" (13.3') on tangents 1 3/16" (15.8') on curves (12" minimum radius) I have used 0.9" track spacing in yards (12') successfully. Most of my freight cars are shorter than 50'. With a 12" minimum radius I chose to limit my rolling stock accordingly.
