Not exactly building a route in Google Earth, but there is one that helps building in the RW Editor by using Google Earth images: www.rwdecal.co.uk I think it's useful for remodelling real railroads, because you can lay and plan your tracks and routes from the real map data. Or well, at least if the image quality of your desired region is good enough.
Web42RW blog - presenting my current projects for RailWorks
I have been building a route in the UK which was dug up in 1965 and using Google Earth and RWDecal I was able to follow 99 pct of the track with no problems at all. The trackbed, berms etc were still clearly visible and there was no need to use markers for the track at all.