Here is a 4x10 foot layout for Fastrack for two trains. The outside mainline is O44.5 (compound) diameter using O48 and small sections of O36. The inside mainline is O36. All other curves are O48 and wider diameter. Mainline turnouts are generous O60 and O72 wye, with three O36 turnouts serving the industrial spurs, and two O48 turnouts at the bottom corners to create an optional connecting RR and a short interchange track. There are cross-overs between mainlines with O60 turnout pairs, and the figure-8 route in the inside creates reverse-loops in both directions. The minimum point mainline center rail separation is 4.25", which has been constructed and tested (discussed on another thread) and will run the SCALE Lionchief Plus SD60M, the LC+ Berkshire, and compressed RailKing MTH SD70.
The layout is for train running of conventional and LionChief/+/2.0 and MTH capable of running O36 (or the O44.5 compound). But the layout also has some fun switching opportunities, and could even be run like John Allen's timesaver switching puzzle.
I have begun construction with the double-track mainlines, and have been running trains with the above mentioned locomotives, and also with Menards double-stack container cars (a set of five). The double-track mainline looks particularly good to my eyes, and I have oriented the long straight section at the front for train viewing. This puts the turnouts for the cross-overs at the top of the layout, usually less than ideal in practice due to the risk of derailments outside of convenient reach... but my construction rolls out .
The layout is constructed with a 4x10 layout surface within an outside frame of 1X4s. If constructed with 2" foam sheets as I have done, creates a lightweight and moveable, or even temporary layout. Mine is on wheels to pull out for 360-degree access.
I have highlighted the O36 track sections in a rusty-red color.