Where does the power come from?

Where does the DC power come from to run all the trains on the third rail portions of the system? I know there's a basement full of power conversion gear under GCT. But, does that power the whole system, or just up to some point and then there's feeds (and AC/DC conversion) from the local electric utilities?

Same question about the AC for the overheads.

if you look along right of way you see a substation roughly every two miles, it has transformers, rectifiers and section circuit breakers.

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All the power used in NY City and lower Westchester County is feed off the Con Ed power grid,part of the Harlem line(Katonah to Southeast )is
off the NYSE&G power grid.
For the New Haven Line,Con Ed feeds,Conn/NY State line to Woodlawn,and Shell to near CP Gate on the Hellgate route,
CL&P and UI Feeds State line to the Fair st phase break in New Haven.
Back in the 25hz era the power plant at Cos Cob and a few Rotary converters were used,1 was at West Farms(Con Ed),
1 was Cos Cob (CL&P),not sure if the 3rd one was off the UI grid near New Haven

Way back in NYCRR days, the railroad powerhouses were at Yonkers and Pt. Morris.

There is a shared sub station now right at the new haven yard across from the new haven PD