Splitting a curve

Hello everyone. I hope i'm writing to the right place. I'm very new with grasshopper, i've just learned about some little basic commands.

 I'm preparing a handrail modeling and i need it to be flexible for user to be able to apply the model to variable surfaces. Based on just a single curve, i've completed the handrail. However i couldn't made it based on a curve which is duplicated from a surface. The surface will be the main deck of a motoryacht.

What i need to do is duplicate a surface border and split it (or by the help of any command, pick a desired length and region of the curve) with adjustable surfaces or points etc. (Simply creating a surface on gh and using "set one surface" over rhinoceros is enough for me to define the surface) But i dont know how to split a curve depending on parameters (Actually i dont even know how to do splitting). I have tried to create 3 breps whose position is adjustable by number sliders to split/trim the surface on the motoryacht but seems like it doesnt work, probably due to lack of my knowledge. 

I'm adding both rhinoceros and grasshopper files here. Handrailtrial.gh is to show you what kind of thing i am trying to prepare. Open the Form2.3dm file and then open dupborder.gh on grasshopper. If you have any suggestions to share, i would be glad to hear them.

Thank you !