Which term describes a unit propelled by any form of energy used in train or yard service?

The term that accurately describes a unit propelled by any form of energy used in train or yard service is "Engine." In the context of rail transportation, an engine is a powered unit that can include various types of locomotives designed to pull trains or perform other service duties. This term encompasses not only traditional locomotives but also other propulsion systems, whether they are diesel, electric, or even hybrid in nature.

In contrast, "Equipment" refers to a broader category that includes various tools and devices used in rail operations, but it does not specifically denote the powered aspect. "Track Car" typically refers to specialized vehicles used for maintenance and inspection tasks on the railway tracks, and "MU Passenger Car" refers to multiple unit cars that can provide passenger service but do not independently provide propulsion. Thus, engine is the most precise term in this context and best fits the definition provided in the question.