It was the US Railroad Administration that ordered the B&O trains routed into Penn Station in 1918. IIRC they were there until 1928 when rate was raised prohibitively high and B&O returned to New Jersey. Lehigh Valley through trains were routed into Penn Station a short time after B&O, but LV stayed until the end of their passenger service, in meantime PRR obtained almost complete control of LV. New Haven came in also with trains for PRR to continue. Long Island was there too, but they became a PRR subsidiary until 1950s.