J.C.,
On my new layout, I have Atlas Ø108 and Ø99 curves with 4-1/2" spacing throughout my dual main tracks and this allows my MTH Premier Big Boy 4-8-8-4 to run together with all the equipment I have except with the MTH Premier DDA40X. If the DD40 is on the outer curve and the Big Boy on the inner curve, they will hit. The Big Boy sticks out about 2" beyond the outer rail, and the DD40 is the worst for taking up space on the inner curves.
On page 3 of my layout thread (link in my signature, below) I posted pictures of the Big Boy and a passenger car. Unfortunately, that was in the old forum, and the picture can no longer be enlarged.
Bottom line: the 6" is probably a safe spacing.
You also have to watch for clearances for tunnel portals, buildings and other obstacles. I have found that if I use the Union Pacific recommended track clearances, everything is OK. That is, no less than 18-ft (4-1/2") total, or 9-ft (2-1/4") from center of track to obstruction. For buildings, etc., the clearance distances are greater.
REF: Page 6 of this document (link)
http://www.uprr.com/aboutup/op...ded/track_design.pdf
From here:
http://www.uprr.com/aboutup/op...cs/track/index.shtml
Good luck with your project.
Alex
