Once you have instrumented your layout for transponding, you simply add transponder equipped locomotives and you are ready to go. Digitrax decoders have been shipped with transponding included since 1999. All current production Digitrax decoders are transponder equipped. Older Digitrax decoders models without transponders and decoders made by other manufacturers can add this capability by installing a transponder such as the TL1 or TF4 to existing DCC locomotive installations. These transponders also add functions to the loco.