With 20-feet by 20-feet, I think the layout would look better and be more interesting with two levels of track, rather than one, and they could be made to be a different railroad on each level - and not necessarily connected. I would not do more than two mainline tracks on the lower level and only one or two on the upper level. That would leave you enough room for convincing scenery (hills, valleys, tunnels, rivers, bridges, a town or industry) and one spur on each level. Don't let the layout get overwhelmed by too much track. The real world has lots of things other than railroad tracks. Just my opinion.
I see that you are a New York Central fan so I can understand why you might consider a four track mainline on just one level. If you do one level, what about two or three tracks and a compressed Harmon Yard. Also, don't forget the Hudson River. A riverbank and some water with track running alongside would look realistic.
MELGAR
