Framing depends on the effect you want to achieve. I often prefer a people's point of view “to scale”, ie the camera being horizontal, the center of the picture is approximately 20 mm above the “ground”, which corresponds to about 1.70 m at HO scale. I choose the focal length as short as possible to give a certain dynamic, thanks to a more receding perspective.
Car transporters TA60 L.S. Models ref. 30191; images
processed by Helicon Focus.
Click the image to see it in higher resolution.
Here, the actual focal length is 18 mm (equivalent to 27 mm in 24 × 36). Note that the background has been digitally processed. See further.