You will find the data you need at Harrisburg, PA as all the drawings which are not in Texas or Washington (Smithsonian) are in the State Archives
Review Document listing 422 and 427, poss. others.
[www.phmc.state.pa.us]
[www.phmc.state.pa.us]
You can back out from these sites to review the other holdings and hours at the archives. This is not the State Library. Broadbelt photos are in the Archives holdings but are at Strasburg, in the RR museum.