With structural failures found in one-third of its train fleet, SEPTA officials announced Monday that the Regional Rail lines would be running on a modified Saturday schedule during the workweek and urged Regional Rail riders to shop for other ways of getting around.

Repairs could take the rest of the summer, but riders who account for 150,000 trips on Regional Rail each day likely face crowded trains and big delays.

Depending on the line, said Ron Hopkins, SEPTA's assistant general manager, capacity could drop as much as 50 percent.

In the meantime, most trains will be running with five or six cars, Hopkins said.

"Passengers should expect crowded conditions," the transit agency said in a late-day statement.

Regional Rail tickets will be accepted on other forms of transportation, he said.

SEPTA said it would boost service on the Market-Frankford and Broad Street Lines and on buses and trolley routes. The agency said it would try to have trains available every half-hour during peak commuting periods.

It also said additional parking spaces would be available near some key transit stations. For example, SEPTA said it has secured 1,600 spots at the former Naval Hospital, at 16th Street and Pattison Avenue, near the AT&T Station.

The structural flaw, a crack in a weight-bearing beam on a train car's undercarriage, has shown up in almost all of SEPTA's Silverliner V's, the newest trains in its Regional Rail fleet.

The agency spent $274 million on the cars and they were put in service from 2010 to 2013.

It was unclear when rail service might return to normal.

"Unfortunately, it will be rough on our railroad customers," said Jeffrey Knueppel, SEPTA's general manager.

Knueppel said the flaw was discovered early Friday when an inspection of a car at SEPTA's Powelton yard showed a 10-inch crack in a 350-pound equalizer beam, which transfers the weight of the car to the axles.

Engineers later found cracks in all but five of about 100 Silverliner cars. All of the Silverliner V's were pulled from service, as required by the Federal Railroad Administration.

The cause hasn't yet been identified, nor has a solution.

SEPTA said that it would attempt an interim fix by welding the beams but that all the equalizer beams might have to be replaced.

Knueppel said that if a temporary fix doesn't work out, service disruptions could persist during the Democratic National Convention, which begins July 25.

However, Knueppel noted that most conventioneers likely would be relying on subways.

215-854-4587@jasmlaughlin