Adding Choke to controller

Hi all I have several controllers with variuos controlls ie brakes, sensitivity, traction control etc but would like to add a choke switch preferably on the wire cables so that I can quickly change the max voltage for a technical section of track. Ideally also with a dial to dial in the desired voltage for the switch to use (ie 9 or 10 volts).

Are there any parts that I should consider that I could get from the local electorinics shop.

Regards

Ray
Team Thunderbird