Cant select curve on surface

Hi, Im trying to fill, enclose these triangular shapes with the ‘sweep 2 rails’ command but cant select the curve on the top of the side(bottom of triangles) to split it. New to Rhino so any help appreciated cheers.

can you link your file? (drag onto reply window)

Breath belt rhino model.3dm (1.1 MB)

Try that thanks.

It’s not working because your surfaces don’t intersect properly.
If you want the triangular sections to be flat, you can easily create the surfaces with the surface point command, click the three edge points and enter… done.
If you want them to follow the curve of the sphere/domed surface they rest on, you’d need to rebuild your surfaces so they intersect properly.

In this file, I offset your circular base curve, extruded to create a trim surface- and trimmed down all your geometry so it intersected. Saves rebuilding, but not exactly where you had your line- if that is important.
Breath belt rhino modeltrimmed.3dm (3.9 MB)

Awesome, cheers for your help. So were the triangular channels not intersecting with the dome?

yes exactly- you can see from top view the triangular sections are straight edged, where the dome is obviously curved. So the triangular parts either need to be rebuilt to go past the dome and then trimmed, or as I did- trim it all down so you make the straight sections align to the dome. Hope that makes some sense.