Last month, we discussed the Reading Railroad’s ambitious plans for placing their City Branch below ground level. One part was the construction of a tunnel beneath Pennsylvania Avenue. The tunnel was to be 2888 feet long and of sufficient width to hold four tracks, two for the main line into the city and two for storage.1 At the time, steam power still ruled the rails, so providing suitable ventilation for a tunnel of this length was not a trivial engineering problem. Extensive correspondence over the issue survives in the Reading archives. Ultimately, the problem was solved by placing a series of ventilating grates down the median of Pennsylvania Avenue above, much like Park Avenue in New York City.
 Engineering department notes, Reading Company Collection, Hagley Museum and Library.
 Scranton, P. & Licht, W. Work Sights. Temple University Press, Philadelphia (1986), p. 182.
 #234 The United States Standard Screw Threads (1864) American Society of Mechanical Engineers. Accessed May 25, 2009.
 Workshop of the World. Oliver Evans Press, Philadelphia (1990), pp. 5-43–5-44.