I've done some research, and I think I know why the white lines are placed as they are.
Turns out trial mountain was not initially thought of as a race track, but as a normal mountain road. So the areas outside the white lines are "roadside", and the yellow striped areas are emergency stop areas and control areas for police etc.
up until gran turismo 4, it still had the partially filled middle line, separating your lane from oncoming traffic.
This lead me on to this:
I know TM is a fiction track but what country would you say it comes from? I went for Brazil in the GTP location I update every week depending on track we race.
We are looking for a place with lots of mountains. Straight away my mind brings me to japan for obvious reasons, but after some thought, I remembered that creature we saw at the lake in GT5 than was rumored to me Loch Ness.
Now, the most believable explanation I found is that it is a reference to the movie "The Water Horse", simply because there is also a little raft with 2 people on it, and the "monster" is far too small in comparision.
Either way, "The Water Worse" is set in Scotland, and Scotland is a very mountanious place.
The scenery from trial mountain is very similar to that of the scottish highlands. It is most likely what the developers had in mind.