I suppose it depends how fussy you want to be.
I built a small prototypical area of a local demolished rail line, but it just didn’t look right. Rebuilding it correctly, at 180 deg rotation, made the world of difference, purely because of the lighting.
The position of the sun, as it rises and sets, is obvious when it is in view. What is less apparent and much more subtle, is the way in which Trainz handles the shading of placed assets when the sun moes during the "day", even when you can't see it. The sides of buildings, for example, will be highlighted quite differently as the sun changes position – at least it does if the model has been properly constructed. You can see this if you move the clock dial in the world flyout.
Here’s an example of a very small change in time where you can see the changed highlights on the walls of the yellow building. They are particularly noticeable on the chimney posts and the indented bit of vertical wall between the balconies.
However, if your route is largely underground I don’t suppose it will make a great deal of difference, unless you are building a lot of surface detail.
Cheers
Casper