rdeal
I checked a circle of O27 track. Diameter, outside of tie to outside of tie across the circle measures 27 inches.
Roadbed.
Traditional:
painted board
green felt stretched over board
carpet laid on board
cork strips under track
Modern:
extruded foam
indoor/outdoor grass carpet
commercial roadbed
Whatever you choose to use, is the best.
It's like Chevy or Ford, which is better?
Dave