The deck height of my layout's elevated loop is about 8". This allows for a 6 1/2" height clearance underneath, enough for even electric locomotives with raised pantographs to pass without issue. As you can see, the compromise is that it does not allow for the best visibility of the scenery underneath.
If you intend to run a ramp between the upper and lower loops, each extra inch of height requires the ramp to be considerably longer. The ramp into my elevated loop rises 8 inches over 12 feet (all of the curved sections are level), producing a moderate grade of about 5%. Most of my locomotives can handle this with short trains.
