A commuter checks the train schedule board in New York's Penn Station Tueday. (Mark Lennihan/AP)

Amtrak spokesman Cliff Cole says engineers are investigating what caused the low-voltage dip at 7:15 a.m. Service was restored at about 8 a.m. NJ Transit had to stop its Northeast Corridor and North Jersey Coast Line trains.

The problem apparently wasn’t related to a transformer fire that halted service on the Northeast Corridor for several hours on Tuesday. The two incidents this week underscore problems created by aging infrastructure on the heavily traveled Northeast Corridor. Cole says Amtrak has been making improvements to substations and other electrical equipment for several years, but some of the parts date back to World War II.