Hi Graham,
My comments would be that the way you view and frame the layout is a key thing, i.e. if it is an exhibition layout, then creating a proscenium - narrowing the field of vision -might help to improve the illusion of depth, by controlling the perspective, focusing the eye on the trains, whilst at the same time making best use of the vertical skyscrapers but not actually needing to model the tops!
I hope that makes sense! Can anyone tell I'm an architect?!
See Catcott Burtle and Much Meddling for egs, albeit in a rural setting.
Regards,
Matt