When a locomotive is described as being "dead attached", this means that it is not online or contributing to the haulage of the train. This is commonly used to transfer locomotives from one depot to another, and is done to avoid wasting fuel when the additional horsepower is now required. Inoperable locomotives are typically transferred in this manner to/from repairs.
Example: 3128 empty fuel train was hauled from Cootamundra to Sydney by X51, X50 and X52. 48114 and 4886 were hauled dead attached.