"It was done as a precautionary measure. None of the trains lost power," said Cliff Cole, a spokesman for Amtrak, which operates the tracks used by both agencies.
The source of the problem was not immediately located, he said.
The agencies had no immediate figures on the number of trains or passengers affected.
Joe Dee, an NJ Transit spokesman, said delays ranging from 20 to 30 minutes were reported Wednesday evening on the Northeast Corridor, Midtown Direct and North Jersey Coast lines, down from 90 minutes earlier in the day.
"The trains are running, but we're still dealing with delays stemming from the congestion from earlier today," Dee said.
However, all the trains were expected to be back on schedule later in the evening.
To help deal with the delays, PATH was cross-honoring tickets for passengers boarding at Newark Penn Station and Hoboken Terminal.
Voltage dropped to about 9,000 volts, from a normal reading of about 12,000, at about 12:20 p.m., Cole said. Power was restored about 1:50 p.m., he said.
A few Amtrak trains that were in tunnels were moved to stations, Cole said. NJ Transit spokesman Dan Stessel said its trains were moved to stations and held there.
The Southeastern Pennsylvania Transportation Authority said its train line between Philadelphia and Trenton was affected.