Hierarchy classification:

1. top of the aat hierarchies
2. Objects Facet
3. Object Groupings and Systems (hierarchy name)
4. systems (equipment)
5. [systems by function]
6. infrastructural systems
7. transit systems (infrastructure)
8. public transit (infrastructure)
9. [public transit by form]
10. rail transit systems
11. rapid rail transit systems

Anotations

Scope note:

Underground railway systems used to transport large numbers of people within urban and suburban areas typically by means of trains comprised of a number of cars operated on the multiple-unit system. Subways are usually built under city streets for ease of construction, but they sometimes must pass under rivers. Outlying sections of the system usually emerge aboveground, blending into conventional railways or elevated transit lines.