As usual, Jim is pretty much on target. I will add that the NMRA's guidelines are just that, and you may want a somewhat larger separation (probably not smaller). I know you are talking mainlines, but for sidings and terminals you want to consider the ease of getting the ol 0-5-0 switcher in there to handle derailments and such.
ENJOY,
Mobilman44
ENJOY !
Living in southeast Texas, formerly modeling the "postwar" Santa Fe and Illinois Central
Dave,
How big is a dog? A lot depends on what scale you are modeling in, and what will be the minimum radius of your curves. The NMRA has some pretty good charts you can use to get started with. Here is a link to Standard S-8:
http://www.nmra.org/standards/sandrp/s-8.html
As you can see, the sharper the curves are, the wider the center to center distance is needed for good operation. I would not use this as 'bible', but it is a good start. Lay out some flex track and make a few tests with those long passenger cars on both the outside & inside tracks to verify what you will need for good operation.
Jim Bernier
Modeling BNSF and Milwaukee Road in SW Wisconsin
My layout is going to be large enough that I can easily run dual tracks on the large mainline L-shaped loop. However, I have a question about clearance between the tracks themselves. In a prior post last year I mentioned that I will be running the Alaska RR observation cars and I've noticed that they take curves very differently than standard rolling stock. I think it would look odd to have the tracks close together except in the curves, so I'm just going to set a gap and go with it for consistency. The question then is what should the gap be to insure the cars don't contact anything - especially trains on the other track. Ideally I would like to actually watch the cars in motion and build accordingly, however I would prefer to lay all the roadbed at once, so I need to set the gap from the outset.
Thanks everyone!