There is little that is remarkable about this except it was so easy to make it work perfectly in an industry where far too often one manufacturer's equipment does not work that well with stuff made by others.
I'm continuing my adventure of refitting of my layout with Atlas track. Two loops have been completed (including ballast, scenery, and I am halfway through loop three - about 90 feet of track in total - a "dogbone" with reversing loops at each end. I wanted dependable auto-de-railing turnouts on both end reversing loops, so I deciding to go with Fastrack switches, since they have a built-in auto derailing features and my considerable experience with them indicates they are bulletproof. I got mixed reviews about the durability of Atlas switches and one has to fit the optional auto derailing kit to each switch, so it was an easy decision.
I expected to have to fuss and fit and custom-work up precise transitions from Fastrack to Atlas and back (top of photo) in order to make the transitions smooth for my loco. No. I used Fastrack to O gauge tubular transition pieces (6-12040) at each transition (three per turnout). I pulled the pins meant to go into the tubular track end and substituted Atlas "Transition Joiners for tubular track" (Atlas item #6095). The Atlas and Fastrack fit perfectly - I tried three of for locos at various speeds and all made the transitions at any speed without any sign of disturbance.