The endpoints are located on terminals, manifolds, or fittings which may be located differently.
The tubes or wires often run at other than right angles.
Right now I have a 3D curve that traces where the centerline of the tubing would be, and it changes easily as the fitting locations change position or angle, but I cannot make a tube out of it unless I copy the curve to another sketch from which I make a model of the tube, then bring the tube into the assembly. Since there is no link between the assembly sketch for the tube centerline and the tube model, the model must be redone over and over as the fitting locations are varied. I am not using splines.
The 3D sketch in the assembly gives me tubing lengths, but I'd like it to show interference without having to model every slight tube variation.