What type of signals did the pennsy use in 1926?
Location, location, location! Seriously the area you are interested in will have a lot to do with the type of signals used. Before position lights semphores were the standard BUT if you are interested in lines purchased it might be anything so Location, location, location still rules. 
Most folks know my bias toward the NYC, but the RR that can’t be mentioned has a great fascination, too; if only because of the sheer volume of its operations.
I have always thought that the greatest oxymoron probably in the history of American railroading was the PRR’s slogan “Standard Railroad of the World;” whatever segment of the P Company you examine; with the possible exception of their engineering dept.
That said, I heartily agree with what the previous posters have written. There is a book, I believe o/p, titled Pennsy Steam and Semaphores (or something close to it) that will give you some insight into PRR signaling. IIRC, it was written by Fred Westing. I’m probably mistaken here. I’ll check on my shelves and get a proper citation.
BTW, I had a friend on the C&O who called them “sefamores.” I find it hard not to call them that myself.
work safe
Try a copy of “Trackside on the Pennsylvania” by Jeff Scherb and available from Highlands Station, Inc., 2600 S. Parker Rd., Suite 1-211, Aurora, Co. 80014-1601.They also publish Model Railroading magazine. The book contains plans for signals, structures, bridges, and signs. There is also some explanation on the different signal aspects.They also publish a book about all the classes of PRR hoppers. For the most part,by the 1930’s, the PRR was standardized on the use of position light style signals for all of their mainlines and heavy secondary routes. It wasn’t until the early 1990’s that Conrail began replacing Pennsy signals with NYC/ RI style “triangle color-light” signals. Norfolk Southern has continued this program since the merger,as at least two PRR signal bridges have fallen on the Pittsburgh Line in 2003.However, there still are many origional position lights still in everyday use on the former Pennsy in PA. I would assume that as the signals wear out or become too hard to repair, they would be replaced. For example, the westbound bridge at CP Rade near Greensburg,Pa, was replaced not because of bad signals, but because the structure was rusted away and in danger of falling down. The matching eastbound bridge is still standing. Good luck and stay safe.