Model Railroading > Track separation

On a double track mainline, is there a standard distance between tracks and how would that be measured? Outside rail to rail, tie to tie, or something else?
Obviously there must be a minimum but what would that be?  Thanks in advance!

Center to center

Railroads use the centerline of the track to define all horizontal geometry including center-to-center spacing. I’ve worked on projects with spacing of only 12’-6” on up to 25’. 13’ to 15’ is pretty common on most of my projects.

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"Traditional" spacing is 2" center-to-center in HO Scale, 2-1/2" on curves. However, the kind of cars you are planning to run plays into the equation. Obviously, if you are doing an era with longer cars, a little more spacing, especially on curves, will improve appearances. Personally, I went 2-1/2" on straightaways. Makes re-railing a bit easier (not that anyone would ever have to re-rail) :-)

TJY

Also, if you are doing a western railroad the spacing can be more generous.

'Cause of our clear clean wide open spaces!!

Bill Deaver
Tehachapi, CA

Track spacing can be subject to State regulations, it has to do with the safety of employees who must walk between or ride on the sides of cars on adjoining tracks.  Unions had a lot to do with getting these safety regulations passed.  
Yard tracks typically have a little extra distance between the first track in the yard and any parallel main track so switchmen in the yard won't be ambushed by a main line train at track speed.