Tom,
I have double line most of the way around my layout. The width varies.
Crossover Inside rail to inside rail is 9 inches.
10/12ft curves.... separation varies from max of 7inches to min. of 5in., inside to inside.
At location of UP-FA, the tracks are about 7inches apart, inside to inside.
Track in this area varies from a minimum of 5inches [going around a tree] to a maximum of 7inches, inside to inside rail. [Hedgeapples fell just moments before train entered area. No time to stop.]
East end loops ~ 6inches between inner rails, narrowing to minimum of 5 inches where the orange reefer is located.
[SD-45 now operated on the New River and Western RR]
My longest cars are Streamliners and SD-45s. There are no problems with them. However, I'm not sure how a pair of opposing direction 1/20, K-27s would do on my narrower locations. My suggestion is to stay at 7-10 inches, inside to inside rails, to play it safe.
Jim Carter