Mating lionel and atlas tracks??

I did it this way: use the Altas-to-traditional track end clamps (oe one side they are like the normal clamps that slide ont the ends of the Atlas track at joints, on the other a round pin as goes into the end of O-27 track), then put those pins into a O-27 to Fastrack transition piece.

I do this six times on my current layout, which is all Atlas track except for two Fastrack switches.