Pardon me for picking a nit, but there never has been any railroad-related structures or facilities where the MSB now sits.

Broad Street Station's headhouse sat on what is now the sunken portion of Dilworth Plaza; Broad Street originally ran right next to City Hall's west wall (equivalent to a mid-block alignment between Broad and 15th just as East Penn Square is aligned with Juniper Street, the midblock between 13th and Broad). 15th Street ran in an underpass beneath the east end of the Broad Street Station trainshed, most of which occupied the site that now contains 1515 Market (Two Penn Center) and 1500 JFK Boulevard (Three Penn Center) -- these two structures being the first buildings erected as part of the PRR's redevelopment of Broad Street Station and its viaduct into the Penn Center office complex, in 1955.

I believe the MSB site contained random office buildings.

__________________
Sandy Smith, Exile on Market Street, Philadelphia
“Basically I figure guns are like gays: They seem a lot more sinister and threatening until you get to know a few; and once you have one in the house, you can get downright defensive about them.” --Theresa Neilson Hayden