If you want that kind of precision, I recommend using a spline. There are plenty of ballast splines that you put down like a road. I even made my own by using the YARN roads. I disabled traffic and put my own textures on them. They are great for thick surfaces such as cobbled roads, dirt or other textures on them. Being thick, the track can be buried in them without the rails sticking up above the surface. You can do this with thin roads too or download one of the many ballast splines from the DLS.
In other words, there are more than one way to beat a dead horse and this is an easy one.
One other thought...
The grid is there for a reference for placing fixed objects and I doubt it was ever meant as something to be used for painting between lines. The grid texture we have today is far different than the one found in UTC through TS12 and T:ANE. The grid is a texture and PBR one at that.