Most of the old steel companies put their name on their products. Structural steel members are made by passing hot metal between sets of rollers that squeeze it into shape; it’s easy enough to engrave letters on the roller surface so that they make raised letters appear on the web of an I beam, or the flange of an angle. The angle in the photo above was rolled sometime around 1930 by at the Pencoyd Iron Works in Pennsylvania. Pencoyd was absorbed into the American Bridge behemoth in 1900 and then into the bigger US Steel behemoth two years later.
That angle is part of a built-up H column in the 110th Street Station on the C train. That section of line opened in 1932, which is why I’m guessing 1930 as the approximate rolling date. The web is a bit harder to read because of shallower letters and more layers of paint
but that definitely says Carnegie. The Carnegie Steel company formed the core of US Steel when the bigger corporation was formed in 1901. In other words, both pieces of steel came from the same corporation, but from different branches that had once been competing companies.
One last note: the rivets are from the original assembly of four angles and a plate into the H shape. The bolts at the bottom of the picture are holding reinforcing that was installed during repairs or renovation of the station, most likely during the major retrofit that took places a couple of years ago.