Train service along the Northeast Corridor from New York to Washington was slowly returning to normal Saturday following a morning service disruption caused by an Amtrak low voltage problem.

It’s not yet clear what’s causing the problem, which was discovered around 9:30 a.m. Amtrak crews were working to resolve the matter, and normal service had returned to the New York and New Jersey region by noon.

But some service problems remained in other affected areas, including Philadelphia and the Washington, D.C. areas.

It wasn’t immediately clear how many riders overall were affected by the service disruption.