Philadelphia puzzle

Two questions were puzzling us in Philadelphia. Just where was the original Baldwin Locomotive Works and why were the railroad tracks nearby running in a trench below grade.
Research pointed to the works being in several locations and in a sense it was, as it covered four huge city blocks, bounded by Broad Street, Callowhill Street, 18th Street and Spring Garden Street (the yellow box shown below, the hatched green box adjacent being a commemorative park, which may also have formed part of the complex).

The rails were sunk below grade about 1900 eliminating many level crossings, as indicated on the tunnel arch (top), shown in detail below.

Pics by Annette.