TrainsForOwen,
Hello & Welcome to the Forum!

Track is measured to the center, between the two outside rails.
Using O-36 FasTrack, you add 2 inches to the radius to accommodate the rail bed.
ex: a circle of O-36 FasTrack has a 40 inch foot print.

The Polar Express Passenger cars appear to slightly overhang the rail bed,
so an extra inch on each side of the rail bed should give plenty clearance.

Here's a video that shows a P.E. operating on FasTrack.


Happy Rails!
Dave