Surviving structures from the Cambria Steel plant form one of the best-preserved sites from the “early modern” iron and steel industry, and have been named a National Historic Landmark. Read more about this here.
The stamp is hard to see, even with raking light, but a rubbing makes it legible:
If you look closely during your subway travels, you can see the same stamp on steel elements in stations along the IRT lines, which were under construction during the same period. You’ll also see the names of other manufacturers, including "Carnegie".
-- Posted by Project Director Chris Neville