I thought it was STC at first as well. But sure enough an STC 1915 frame is very
different than the 1918 frame, while the later PTC frames look more like the earlier
STC frame. Weird!

As for the date, can those spot/flood lights on the scale house help pin down the
decade at least?

Tim O'Connor

That's one of the very early (circa 1912-1916 or so) Pennsylvania Tank Car tanks. The frame design was taken up by Standard Tank Car when its predecessor started in 1916 and PTC changed frames.
David Thompson

http://i.pinimg.com/1200x/1d/e5/18/1de51863456974b82a4b9bf3146c35e7.jpg