String is used on the inside of the rail head. IIRC, the reference point is to be 1/2 inch below the rail head.
When Hesstons 24&36" gauge rail was laid in 1965/66, the contractor laid it like an industrial siding. Ties laid directly on the dirt (no sub grade ballast!), then rails without tie plates spiked down on mixed hardwood penta treated ties. Dumped some stone around the ties (no raise!!!) and called it ballasted. The theory was they would weather white & look great for years. WRONG! Ties settled in the mud over the years, and when we did a contracted rehab a few years ago, found many ties that looked great on the surface, but only 1.5 inches remained of a 6 inch tie. We also dumped over 1000 tons of new ballast on 2 miles of line [with Huckleberry's EBT hopper and a tie to level ballast], and did a 3 to 4 inch raise and tamping of the whole line, while installing about 2000 new creosoted ties. Looks great after 10 years and much more stable now that it has some drainage, though a foot of sugrade ballast would be great.
John Harbeck