Late 1920s. My source was the 20 volume collection of Aero Surveys aerial photos in the Map Collection, Social Science & History Department, 2nd floor of the Central Branch of the Free Library of Philadelphia, on Logan Square. There were enough photos to show the demolition of the roundhouse, building of the 2601 Pa. Ave. apartments, and roofing over the 2600 block of the Subway. There were railroad shops at 2701 Pennsylvania Ave., the leads started in the 2800 block. In 1965 and some years later, there was still a curved retaining wall to keep the backs of the Brown St. row houses from falling onto them.
Another shot showed the turntable near Race Tower, Reading Terminal.
The oldest of these photos was taken in 1893 from a balloon and shows the Junction Rwy., which the Reading used, at Zoo Tower.