The copyright has expired on many old periodicals that are available from Google books, but due to age, the quality is not great. That said, the info can be priceless. I have found interesting stuff in Railway Engineering and in Railway Signaling. You could also search the Library of Congress.
I would also suggest joining the PRR Technical & Historical Society; they put out a quarterly magazine chock full of the kind of info you seek. Their magazine index has kind of fallen behind, but it will help you identify topics you want to read about in a specific quarter and year. Then, the hard part; finding somebody willing to part with their copy.
Good luck.
Chuck
