If you think about it, when we're looking at real trains (from public areas and not on railroad property without permission) we're typically looking up at them. It's a more realistic view to look at the models at eye level or look up at them. Practicality, however, keeps us from setting up a deep layout at eye level as it's more difficult to reach stuff without a step stool. The added bonus of higher layout platforms is the additional storage space below, plus it's easier to work on wiring.
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