MRH had a great article on this in their 1st edition. It works for any scale and any era. It is based on the length of your rail cars. I don't remember it per se but it was something like 3 x length (ie car is 9" then 3X would be 27" radius) will work but look toy like, 4 X would be better and look better and 5 X is best. You can download that issue for free here http://model-railroad-hobbyist.com/mrh2009-01/download as they are forever free for all issues
Modeling a proto lanced SP line in the Klamath River area in 1980 - "The State of Jefferson line"