Alex, as you begin to set up the catenary you will discover that not everything fits perfectly. For example, the circumference of your catenary may not evenly divide by the length of the messenger wires. The expansion plates and extended give you some flexibility to get everything to fit.
Tip: Install one pole then assemble a couple of pieces of messenger wire and attach a pole or two. This will allow you to line up the messenger wire so you can anchor the pole in the right place. Use one of your electric locomotives with the pantograph(s) up so that you can center the wire on the pantograph - this will take into account the overhang of the loco on curves.