HowTo/Build Catenary
From TrainzOnline
To get your catenaries (electric overhead wires) to match your track in an electrified Route, it is generally best to place the spline points directly on top of the track spline points. This ensures that the curvature matches that of the track and makes it easier to match the height.
When laying your catenaries, press Alt-O to show the track spline points then hold the Shift key to allow placement of the catenary spline on top of the track spline.