After two straight days of major delays on Metro’s busy Red line, the transit agency has begun issuing fare credits to scores of frustrated riders.

By mid-afternoon Friday, 550 people had complained to Metro, mostly via the Web, about hours-long Red line delays Wednesday and Thursday. The problems were so big that Metro General Manager Richard Sarles issued a public apology Thursday.

As with Metro riders generally, more than 90 percent of those who complained are holders of registered SmarTrip cards, Metro spokesman Dan Stessel said. As a result, in most cases, Stessel said, Metro is able to confirm that a person angry about the Red line problems actually was using the system when a delay occurred.

As of 3 p.m. Friday, 316 of the 550 complaints had been “reviewed” by Metro and fares had been credited back to SmarTrip cards in nearly every case, typically in the range of $2 to $3, Stessel said. He said the transit agency probably will credit back $1,000 to $2,000 in fares by the time all the complaints have been processed.

He said a voucher for a free ride will be given to customers who have paper fare cards showing that they were caught in the Red Line delays.

On Wednesday, after a cable fell from the Woodley Park station’s tunnel roof, Red line trains heading in both directions were forced to use a single track for three hours, causing long travel delays while a repair crew fixed the problem. On Thursday, brake problems halted a Red line train between the Fort Totten and Brookland-CUA stations, resulting in more single-tracking and more frustration for rail commuters.

“We don’t guarantee that every train will arrive at a certain place at a specific time,” Stessel said, explaining that Metro normally doesn’t issue fare credits to riders who are slightly delayed in getting to their destinations. But in “egregious cases where people have been extremely delayed, those will often qualify for an adjustment,” he said.

He said Metro has issued large numbers of credits in the past after long rail delays.

“This isn’t a first,” Stessel said, but added, “I’m not sure we’ve done it on this scale.”