I found a very detail file (pdf) that give a very extensive knowledge of real track versus a G scale network.
A lot of details are base on real line, the design, how to move goods train, etc.
It's in french...
But its a really good explanation, I hope you will find it interresting, there's 170 pages with a lot of clear schematics and pictures.
File in PDF
Edit : the title is about HO, but when you read its about G scale
Here the link of the website, there's a lot to see and good stuff about steam engine :