You really have two separate issues here:
Since I run analog DC, I once considered powering my motors through the catenary. After considering reality, I decided to:
Ease of maintenance and uniformity of control trumped, "Operating like the prototype." The support structure, substation and distribution wiring will be there, but the actual catenary will be imaginary and the pantographs will be fixed at a reasonable height - low enough to clear anything, high enough to look as if they are drawing power from catenary that isn't really there.
Chuck (modeling Central Japan in September, 1964)
All you need to do is run one of the decoder's voltage input wires through the toggle switch. To run the locomotive through the overhead catenary, one side of the DCC booster's output must be fed to the catenary. It seems to me that sparks created as the pantograph rubs on the catenary would really raise havoc with a decoder, though.
I've never heard of anyone using overhead catenary with DCC and that topic has not been brought up before during the time I have been on the forum. Most modelers probably leave the locomotive wired for track pickup and have the pantograph only for show, with no power connected to the catenary.
Are you really planning to use an active pantograph? Why?
A toggle switch is a toggle switch. The center post goes to the decoder. One of the other posts goes to the pantograph, and the other goes to the non-common rail. This assumes that you're using a single-pole, double-throw switch, and you still plan to use normal two-rail DCC for other engines.
Another possibility is that you run only pantograph / overhead units, and that both of the rails are common/ground. In this case, you would want a double-pole, double-throw switch. In the pantograph position, you would want to combine the feeds from the two rails so that you get pickup from all wheels. In the rails-only position, you would open that connection. If you do this, and you inadvertantly put the unit on a normal rails-only layout (or a rails+pantograph dual-mode layout) with the switch in the pantograph position, you will have a dead short. If you put the unit on a pantograph-only layout with the switch in the rails-only position, on the other hand, it will just sit there and not run at all.
Really, though, you don't want to deal with live-overhead. The big advantage of doing that used to be the ability to run one train off the overhead and another off the normal DC rails. With DCC, though, you can control all of your engines independently through the rails alone.
It takes an iron man to play with a toy iron horse.
If one happens to replace an original DC board with track/pantograph toggle switch that came with an HO Electric locomotive prototype with a DCC replacement board, how can you add or wire a track/pantograph toggle switch in addition to the DCC board.