I wonder if "dummy" is etymologically related to "deadhead."

dead·head (dµd"hµd") Informal. n. 1. A person who uses a free ticket for admittance, accommodation, or entertainment. 2. A vehicle, such as an aircraft, that transports no passengers or freight during a trip. ... tr.v. ... 1. To pilot or drive (a vehicle) carrying no passengers or freight. .... adv. Without passengers or freight; empty.

In railroad use, I have heard "deadhead" used to mean a freight car belonging to the railroad that is being pulled empty, or an off-duty employee riding as a passenger on his way home or on his way to the place where his assignment will start. In both cases, the movement generates no revenue for the railroad; he/she/it is dead weight, so to speak.

This usage is much older than "deadhead" meaning a fan of the Grateful Dead.