I am not an expert on the technology, but it is not hard to "piggyback" voice communication over an electric power line in various ways:
a) Some college and high school radio stations use facility power lines as their transmission media. A regular radio tuned to the proper frequency will pick up the signal, but the radio has to be in a school building.
b) As mentioned, there are home intercoms that transmit over house wires.
c) Some subway-el systems transmit trainphone over the 3rd rail. I don't think the quality is that good and there is a lot of "hum" in the power background. (Even regular telephone circuits pick up the hum). I believe some systems started off with 3rd rail carrier but switched to true radio, which requires the stringing of antenna through all the tunnels and stations, but gives a better signal and allows hand held walkie-talkies.
An electric wire can carry currents of different frequencies, so AC power current, DC power current as well audio frequency and radio frequency can be all carried on the same line. For example, a RR line powered by AC has both the AC power (25 or 60 Hz) as well as the control signal (100 Hz and others) sharing the medium. Some lines even have multiple power sources from a separate wire, such as DC via third rail. There are "filters" (IIRC, "impedence bonds") that separate out the stuff.
So, from a purely theorectical point of view, voice and data communications can be carried on power lines. How well and expensive that would be in practice remains to be seen.
For the calls above I doubt they used the trainphones, which are meant more for trains. Before SEPTA switched over to Bell, it's internal communication system between towers and the like was pretty bad. That was the result of just physical decay of the private network -- old wiring, open pole wire, old telephone sets and switchgear, interference from power sources, etc. SEPTA dumped it's own system and switched over the Bell Centrex which was a huge improvement.
PATCO's original internal telephone system (a used SxS switch) had a terrible hum in the background since the switch was atop a power substation and telephone lines near traction power lines. It has been replaced and it's much nicer now; presumably the new system has better filters to keep out the power noise.
Well into the 1980s a lot of carriers had handcrank local battery telephones still in use as wayside telephones; many of those phones used antique components.
Keep in mind that railroads and transit carriers almost always built their own voice communications network instead of relying on Bell; it was more cheaper given the distances involved. I suspect the widespread deployment of ESS allowed Bell to offer reasonable rates on a regional Centrex system that wasn't possible before. For example, the Upper Darby Terminal of the Mkt-Fkd line is out of the city limits and it's a 3 message unit between one terminal and the other. Given the numerous times the dispatchers, motormen, towermen, cashiers, supervisors, and shopmen need to communicate with each other 24 hours a day the message unit charges would be horrendous; it's free with a private system. Further, carriers already have trained staff who maintain the traffic control signals and they maintain the telephone network as well.
In the last 30 years, both railroad and transit have upgraded much (but not all) of their systems. Railroads have gone heavy into fibre optic and even satellites. Transit has gone into advanced systems "applique" phones. NJ Transit gives train crews Nextel cellular phones for emergency use and to double as walkie-talkies.
Would anyone know the history of telephone ownership on the New York City subways and IRT, BMT, and IND predecessors? I think whatever private networks they might have had were dumped long before other subways.