This track plan design is for a 12X20 space and running longer trains through wide-radius curves on a double-track main for train-running and watching. Turnouts are O-72 Fastrack and all are reachable. The outer route is O-96, the inside route is O-84. There is a staging yard, two routes for running trains (through North Park and South Park), and four interesting junctions. Trains can switch between the main lines at either the North Park junction or the South Junction.
The junction at North Park is an Atlas-O #5 double-slip switch with flex track approaches, which could be replaced by a junction made by two O-72 Fastrack 'Y's. The double-slip switch allows a "twice-around" route over both main lines. There are no reverse curves through any of the junctions, or anywhere else on the layout.
Not counting the On-30 narrow-gauge, up to four trains can run at the same time using command-control/LionChief: two trains following each other on each main line. Switching the staging yard requires the use of the inside main route, but the outside main can continue running.
The On-30 railroad is Bachmann E-Z track (HO), which comes with Bachmann's sets of fine On-30 trains. It is 18" radius with some 22", and one #6 right turnout.
There are lots of tunnel portals through the Blue Mountains that act as a scenic divider between North and South Parks. Way more than I have used in previous designs. Having the staging tracks go through tunnels is unusual, but could be a fun feature.