What encompasses any unit used in train or yard service that is not specifically a track car?

The correct answer is based on the definition of different types of railway units. In this context, an "engine" refers to the powered unit used to pull or push trains, making it a primary component of train service. Engines encompass a range of locomotives that are designed specifically to provide motive power. They play a crucial role in the transportation of goods and passengers within both train and yard service.

Engines differ from track cars, which are typically vehicles that do not provide motive power themselves but are used for maintenance purposes, or to carry equipment and personnel. The term "equipment" is more generalized and can include various types of vehicles, but it does not specifically focus on power units like an engine does. "Absolute block" pertains to a signal system used to prevent collisions on railway lines and is not related to units of service.

Thus, determining the term that specifically covers powered units in train or yard service leads us directly to "engine," as it correctly represents the category defined in the question.