I checked with an ex-PRR guy who used to work in Columbus, and he checked with someone else who confirmed that the PRR trains operated with cab signals throughout, while B&O trains did not, so there must have been some kind of a waiver. He scanned a few pages from a joint B&O-PRR timetable, which I don't know how to post here, so I'm quoting the pertinent parts below:
BALTIMORE & OHIO RAILROAD COMPANY. /WESTERN REGION / AND / PENNSYLVANIA RAILROAD COMPANY / COLUMBUS & NEWARK DIVISION / JOINT TIME TABLE 27 / April 29, 1951 [in B&O format]
There follow the names and titles of the Supt. Columbus & Newark Div., Supt. Panhandle Division, P. R. R., and Supt. Columbus Division, P. R. R.
12. CAB SIGNALS / Cab signal rules 295 to 298, inclusive, are in effect for Pennsylvania Railroad as follows:
Between 1st Street [sic], Newark, and Neilston, Columbus, on No. 1 westward passenger track.
Between Neilston, Columbus, and First Street [sic], Newark, on No. 2 eastward freight track.
Between Heath and Outville and between Alum Creek and Neilston on No. 3 westward freight track.
Between Neilston and Summit on No. 4 eastward freight track.
Cutting in and testing sections are specified, along with exceptions permitting non-equipped locomotives for shifting and other purposes between U. S. Tower and a point 270 feet east of milepost 181 in East Columbus and between First Street, Newark and Heath.
Note also that tracks were numbered according to B&O practice.