Hey Billy...here is what I did for a transition at my Atlas bridge. Since you want a transition from Atlas to Gargraves / Ross you can make a solid connection using the Atlas track joiners. First, make sure that both track sections are level.
I then took the Gargraves and using a Dremel tool....cut off the bottom "web" of the hollow shape rail just enough that the Atlas joiner will slide right on the bottom of the Gargraves. You need to leave just a very small "lip" for the joiner to grab on to. This is not as tough as it looks. You can solder the joiner for an even tighter bullet proof connection. On my closeup, you can see where I did not do such a good job with alignment on the center rail! The (2) outside rails look pretty good.
For those areas where the bridge will lift out, just make sure to shim the rails to match and leave a small (3mm) gap.
I attached (3) pics to help with the visual. Hope it helps.
Donald
