Is it a powered unit with a QSI decoder? If it is then that is the "magic wand" used to reset the decoder in the event that it becomes unresponsive to DCC commands.
To reset an engine, power the track with about 10V DC and hold the "wand" an inch or two above the reed switch which is usually located near the smokestack. I think there is a verbal message that indicates that the decoder has been reset.