NEW YORK — Federal and state authorities should investigate the utility and transportation authority responsible for running the nation’s second-busiest railroad following a power failure that has disrupted service for tens of thousands of commuters in New York and Connecticut, two U.S. senators said Sunday.

Sens. Charles Schumer, D-N.Y., and Richard Blumenthal, D-Conn., said they have sent a letter to officials at the Department of Energy and New York’s Public Service Commission asking them to help restore power and figure out what caused Wednesday’s power outage at a suburban New York substation of the Metro-North Railroad.

The heavily used line between New York City’s Grand Central Terminal and New Haven, Conn., is serviced by two high-voltage feeder cables. One of them was taken offline weeks ago as part as a previously scheduled upgrade. It is not known what caused the second feeder cable to fail. The outage, officials had said, could take as long as three weeks to repair. On Sunday, they said full service should be restored by Oct. 8.

The Public Service Commission announced Sunday it was conducting its own independent examination of the power failure, and had asked New York-based utility Consolidated Edison, which supplies power to the line, to save all equipment associated with the outage for its investigation.

A spokesman for the Metropolitan Transit Authority, which oversees Metro-North, said in a statement that the agency is confident one normally functioning 138,000-volt cable could carry the full load of electricity needed while the other cable is being upgraded.

Con Edison said its focus is on restoring power to the tracks, adding it would work to determine the cause of the failure at the substation.

The developments Sunday came as the MTA announced it would be able to provide limited electric train service for Monday commuters that, when combined with additional trains running on diesel, would be able to accommodate about half of the line’s normal weekday ridership. The MTA also said more than 8,000 park-and-ride spaces have been created in Westchester County and the Bronx so riders can reach other MTA services into Manhattan.

Amtrak also announced it would offer limited Acela Express service Monday for commuters traveling between Boston and New York for the first time since last week’s outage on the Metro-North line. An Amtrak spokesman said there would be five, rather than the normal 10, Acela Express trains operating in both directions Monday.

Commuters who rely on the Metro-North train service have experienced hours-long delays since the outage, and highway traffic has been bumper-to-bumper in parts of Connecticut as riders took to the roads to get to and from work.

Connecticut Gov. Dannel P. Malloy warned commuters to expect significant delays and urged them to come up with alternative plans as workers continue to repair the line.

The MTA also urged riders to leave additional time for their morning commute and suggested people organize carpools and other alternatives.

Still, said the senators, more needs to be done to ensure a similar outage isn’t possible anywhere else.

“To grow jobs and strengthen our economy, safe and reliable rail service must be a top priority, and it is simply intolerable for a single cable failure to imperil that progress,” Blumenthal said.