Wires went up 1927, came down/de-energized in the McGinnis era of the NHRR. Later the trustees of the now bankrupt NH brought back the wire, mostly from what was on the Danbury branch until approx. 1968 when it came down for the last time. The subs may have changed over time, but at the end there was subs at Fremont/Pond, ENY (fed also by a huge LIRR sub, this one was huge and may have fed into the subway at one time. Anyone have more info on ENY's old sub, please let me know) NO/New Lot's, MJ/Manhattan Jct., and finally 4th Ave Bay Ridge. One of the old farts who posted me when I was new had briefly worked at Fremont before the contract ran out, he recalled working under the direction of the NH load dispatcher at Cos Cob when necessary to sectionialize cat power. I don't recall how much he controlled, but he did deal with controlling breakers along part of the branch, which was unusual for LI operators, normally that was outside the scope of our work.
Cameron, dive into the archives of this site, there were posts from both engineers and conductors who once worked the branch. Try also the forum at NHRHTA, while they will be quick to point out the NY Connecting's territory ran out at Fremont, the NH always controlled the power along the Bay Ridge, either from Cos Cob or Van Nest, and some may know more about the old sub at Bay Ridge. Good luck in your search.
