Rail spacing

Switches will often be set up to be 3.5 and 4" on center in yards.  Meaning that if you buy a Ross switch, you can buy a return curve that will put your tracks thus.  Crossovers too are usually at 4" oc.  Mainlines as well, although with the bigboy sized engines, I will defer to others how they handle the distance between tracks on curves.   

Not really, but there are general recommendations. Keep in mind that compared to prototype, all model railroad curves are ridiculously sharp.

• On straight track, everything but excess-width equipment will clear at 3.5" center-to-center spacing. This is good for yards and even mainlines, but keep in mind that you need room for 1:1 scale fingers and derailments could get nasty.
• Curves are complicated as you need to compensate for overhang/underhang of equipment on adjacent tracks. NMRA recommends 5" center-to-center spacing on curves from 36" Radius (O-72) up to 48" Radius (O-96); 4.5" for 48" Radius (O-96) to 60" (O-120); and 4" for 60" Radius (O-120) and above. The wild card here, though, is if you have large articulated locomotives being used in conjunction with long freight or passenger cars as a Big Boy will more than likely hit a passenger car on the outside track. If you have, or are planning on large articulated steam, then you might want to put in additional spacing. If you're planning on something smaller than 36" radius (O-72) you'll need to adjust spacing accordingly -- testing with the equipment you're planning to use is always a good idea.
• To spread curves out a bit, use of easements will help. On prototypes, they sometimes spread tracks apart a bit on curves this way as well. Flex track is your best friend here.

Atlas curved sectional track and their 2-track truss bridge are designed around 4.5" spacing. This looks pretty good, although wider than you'd likely see on a prototype.

Hope this helps some.

NMRA standards S-7 and S-8 might prove useful,

Thank you very much William 1, AGHRMatt and mwb. Precisely what I was looking for.