Complex sentence: For bow collectors and pantographs, this is done by having two contact wires run side by side over a length about four wire supports: a new one dropping down and the old one rising up, allowing the pantograph to smoothly transfer from one to the other.
Split into simpler sentences: For bow collectors and pantographs, this is done by having two contact wires run side by side over a length about four wire supports. A new one drops down and the old one rises up, allowing the pantograph to smoothly transfer from one to the other.