At one time (c.1915) Altoona was home to no less than about fifteen continuously-manned interlocking plants. These included (on the main line, and in sequence) BELL, ANTIS, FURNACE, ALTO, and SLOPE. Even more were located on what were referenced in the Employees Timetables as the Antis-Alto Secondary Tracks, and included FARM, BRUSH, ROSE, SCALE, WORKS and SOUTH (These probably operated under "yard rules" -- no train order authority except WORKS, which served both the Main and Secondaries.
When I discovered these treasures as a Penn State freshman (1968) only ROSE and SOUTH were still operating on the Secondaries, and the entire operation was drastically cut back not long afterward, but a very accurate description (maps included) survives in the Minutes of something which was called the Altoona Railroad Club (same era -- c.1915) and which are preserved at Penn State's Pattee Library
What a revoltin' development this is! (William Bendix)