Glen-
While the image is partially blocked by the West Grossdale/Congress Park tower, it is clear that as of the 06-29-19 date of A.W. Johnson’s shot of #648 westbound about to cross the northmost girder bridge over the IHB the interlocking signals are still on a cantilever mast.
Charlie Vlk
Charlie:
Another question about Congress Park, not related to the IHB bridge. Does your 1917 photo show signal bridges of any type in the background? Here is why I ask.
The BNSF signal drawing files contain a Valuation Signal Bridge List of the signal bridges installed in 1913-1914 west of LaVergne for ABS. They do NOT list any signal bridges for the interlocking at Congress Park, specifically Bridge 13.11 and 13.34. Between Hollywood and Western Springs, the list includes 4 track bridges at 11.60 and 12.59, and 3 track bridges at 13.67, 14.12, 14.49 and 15.31.
Although I don't have a date for when Bridges 13.11 and 13.34 were placed, I don't dispute the Valuation List, because these two bridges do NOT contain crossframes, and the 1913 bridges all had crossframes. As near as I can tell, crossframes were not used on all new bridges fabricated from 1916 on.
I would like to pin the date down for the placement of Bridges 13.11 and 13.34 (13.37 after 1947), because it is also likely the date when 12.59 through 15.31 were first re-spaced (12.59 went to 12.35, 13.67 to 13.88, either 14.12 or 14.49 went to 14.61 (to 14.35 after 1951) and 15.31 went to 15.49).
One thing that might have occurred is that the Q may have kept the cantilever style signal bracket masts at Congress Park (originally for the manual block signals installed in 1890) for the original interlocking signals, and then replaced them with the signal bridges when the IHB bridge was raised from its original position. I know that bracket masts were used at other locations into the mid 20's, specifically Buda. Presumably there was some sort of interlocking at Congress Park from 1897 on, but employee timetables don't really say when the interlocking was first placed in service.