Nice looking route, I like the use of the Lothian bus reskin!
Re. the grasses, we've had the same trouble while building the Deltic Preservation Society routes. Some of the grasses display this problem, some don't.
The guidance given was to use the S11 range A to T set and JVC (M) for large areas- I've not used this particular range myself so I can't confirm but try one out and see if it makes a difference for you. EDIT: I've checked the route I'm building, the main grass I'm using next to the line is JVC (M) 2g- this one definitely is OK.
HTH.
Thanks very much for the info and comment, I'll try that grass. Ta very much.