I usually build double tracks on my main inter-city lines initially as it to saves the hassle of having to upgrade them later. I find that with single track lines its almost always necessary to upgrade later as traffic grows. (I always start in 1850 on a large map.)
For smaller dedicated freight lines, or short branch lines, a single track generally adequate. Once they are running satisfactorily its not necessary to increase the frequency as traffic grows, just make the trains longer.