I’ve always used the term “transition” for this sort of geometry, an “easement” being an access or right of way.
Working with Fastrack, or any other sort of set track, the main thing is to keep track of the total curvature of the bend, versus the required curvature. Distribute the total curvature symmetrically and ensure that the total curvature is correct, ie that a 180 degree curve adds up to 180 degrees, and there isn’t really much to go wrong. Half-length O48 curves are your friend here, because two O36 = three O48, so you need one and a half O48 to equal one O36 or two O72.
Provided the resulting curve fits into the available space (SCARM or similar is your friend here!) you just need to check overhang clearances.
One general comment I would make is that the first place to look for problems with side-throw clearances, is outside exit curves.
