Most of Digitrax's HO scale decoders come with a nine pin in-line harness connector. This allows the decoder to come unplugged from the wire harness after the harness has been wired to the locomotive. When the decoder is unplugged from the wire harness, this dummy plug can be plugged in in place of it to allow the loco to run on any DC analog layout, or on any DCC layout that allows an analog loco to run.

Many people wire their locos for DCC with a wire harness before they get enough decoders for all their locos. They can then use these dummy plugs to allow them to run some of those locos on a DC analog layout, or on their DCC layouts in the analog mode.

Replacing the decoder with a Dummy Plug also allows you to sell the loco without the decoder.