Which tracks are considered Mainline Tracks?

Mainline tracks are defined as the primary tracks used for the movement of trains over long distances, connecting various locations and allowing for the operation of passenger and freight services. The correct choice underscores that all operating railroad tracks, excluding those found in yards and terminals, are categorized as mainline tracks. Yards and terminals are specifically designed for tasks such as the loading and unloading of cargo, assembling and disassembling trains, and storing railcars, rather than for the mainline transit of trains.

The other options refer to specific types of tracks that do not align with the definition of mainline tracks. For example, tracks in train yards are typically dedicated to sorting and storing trains, while tracks exclusively designated for maintenance vehicles or emergency vehicles serve very specific and limited purposes within a railway system rather than facilitating the general transportation of trains.