Here we melded two reversing loops with two left-hand switches
You can add both O27 switches at once, or one at a time, to make sidings. The uppermost switch leads to a large arc on the left using 42" curves.
Add two switches, track and three 42" curves in lower right corner. You can add one switch at a time, or both at once. (Upper right corner uses 27" curves)
Two more switches make a passing siding. Run long cars on the outer loop, or use this layout for interurban trains, trolleys or small freight drills.