@EricM you appear to be using the words “curve” and “line” interchangeably. Usually in Rhino a “line” is a straight line. A polyline is a series of straight lines connected together. A “curve” is a single spline, usually degree 3. A single curve of degree 3 will have continuous curvature as well as tangency. A “polycurve” is two or more cuves joined together, and does not necessarily have continuous tangency or curvature at the joins. What is your input?
A “line” can be considered as a degree 1 curve but is usually treated separately from curves.
RebuildCrvNonUniform rebuilds lines, polylines, curves and polycurves into single, degree 3 curves with continuous tangency and curvature.
To rebuild a straight polyline will all segments tangent into a single line with two control points use Rebuild with the output Point Count set to 2 and output Degree set to 1.