Duplicate the edges so we get the circles:
Do this for all 4. Then, offset the circles by 2 units outside.
Next, I have created a line between two radii of two upper circles, and from the middle of that line I created another line 10 units long to up.
Then, using another line connect the middle line from the upper end, and using Tangency option in Osnap, finish the line on bigger circle as tangent.
Do that on the other side too, or simply mirror it. Next, using InterpCrv command, make an arch starting from and ending in the tangency points, and the middle of it in the edge of previously created two lines.
Mirror that arch 3 more times, and blend them.
In the center of that (I have created a line that connects two opposite circles (like on the upper image) just to find the center) create a circle of radius 68 units.
Now, select that circle, that arched closed curve, and 4 smaller circles, and extrude them all by 0.5 units.
In the upper image, I have already created a circle of radius 38units. Next, we’ll move it towards inside by 34 points. Then copy and paste the circle, and move it towards outside by 18 units.
Next, using End, Mid and Cen options in Osnap, create a polyline like on the image below:
Explode that curve, and offset the angled part by 0.5units.
Using extend command, extend that offset curve, then continue with line command and create a section like on the next image:
Again, you can either use rail 2 sweep, or rail revolve command.
Fillet the edges by 1 unit.
Next, create two lines, and one rectangle:
Trim off the parts we don’t need, join and fillet corners. I filleted with 1 units left two, and with 2 right two:
Extrude that curve, and rotate by 45 degrees, and mirror it, or copy and rotate by 90 degrees.
Then, trim that . You know the drill…
Ok, again, we will create a little cross section curve. You can revolve it, or use a rail 1 sweep or rail 2 sweep…
After rail 1 sweep: