the 3D box in sketchup tells me, the viewing angle in a full height isometric for simutrans is approximately 30 degrees.

I don't understand it, but half height is approximately 16.1 degrees.

I'm sure someone can figure out the math

From the middle of the base of the 'tile/box', to the top corner of the box is approximately 30deg.

If you only go up to half of the corner, you get 16.1 deg

Graphical illustration with math that I've long forgotten.

oh, right, it's slowly coming back to me... You 'look down', from horizontal, by 30 degrees. So if objects on the ground are slanted at 60 degrees, it will appear perfectly flat to the viewer. The numbers are probably easier to work with if they are expressed as radians. Probably...