The old New Haven was also at 25 Hz. There used to be a locomotive change at Manhattan Transfer (not far from the current PATH Harrison station), however, the original Pennsy New York electrification was third rail (600 vdc) from Sunnyside Yard to Manhattan Transfer in order to enable tunnel operation under the Hudson and East Rivers. When the PRR extended the 11kV catenary from Trenton to New York, Manhattan Transfer was eventually done away with.
Now the former New Haven line is 12.5kV 60 Hz to New Haven (formerly 11kV 25 Hz, the old NH power system was badly worn), with the new Amtrak electrification from New Haven to Boston at 25kV 60Hz.
Thanks for bringing me up to speed. I was drawing a blank on the 1999 Amtrak electrification to Boston.