If you don’t want a board through the middle then creative use of landforms / buildings is a good way to visually separate the two sides.
It doesn’t have to be a massive mountain or a wall of buildings: the suggestion of stuff will cause the mind to “fill in” the rest of the view block, even if your eyes still see trains on the other side!
(In graphic design, when the viewer’s mind completes the picture based on partial information, that’s called “closure”).
My little 23"x41" layout (which is close to the N-scale 4’x8’ equivalent of 2’x4’) has two scenes, waterfront and mine that are separated by some steep mountains & trees:
!(http://i1192.photobucket.com/albums/aa332/mcfujiwara/Mt Coffin and Kalama RR/MtCoffinKalamaRR-planshoulda-revise-5vi11.jpg)
While it’s really a three-sided divider (the track loops around one end out in the open), the corner rockface creates a gorge and acts as a mini-Bellina Drop.
The mountains in the middle are made from 5-6 layers of 2" pink foam which, when rasped & shaped, resulted in a mountain 8"-10" high, and 6" wide (at the most, at the base):
!(http://i1192.photobucket.com/albums/aa332/mcfujiwara/Mt Coffin and Kalama RR/DSC05649.jpg)
!(http://i1192.photobucket.com/albums/aa332/mcfujiwara/Mt Coffin and Kalama RR/DSC05650.jpg)
This looks way artificial (because of the steep incline), as well as easy to see over, but ground cover makes the mass “recede” and the trees add 3"-5":
!(http://i1192.photobucket.com/albums/aa332/mcfujiwara/Mt Coffin and Kalama RR/DSC06209.jpg)
!(http://i1192.photobucket.com/albums/aa332/mcfujiwara/Mt Coffin and Kalama RR/DSC06188.jpg)
Even at this height (45"), I am able to see over the hills & thr