By Gina Cherelus

NEW YORK (Reuters) - The Philadelphia-area commuter rail service abruptly removed a third of its cars from service after discovering defects, but said it has plans in place to accommodate the roughly 50,000 people expected later this month for the Democratic National Convention.

The Southeastern Pennsylvania Transportation Authority (SEPTA) announced on Sunday that it would remove all 120 Silverliner V regional rail cars out of its 300-car fleet after officials found cracks in the main suspension system last week.

"It's still being looked at thoroughly and aggressively by SEPTA and Hyundai Rotem, the manufacturer," SEPTA spokesman Andrew Busch said. "The main fear when it was discovered was that the cracks could grow and impacted the train's potential for a derailment as the train travels at a higher speed."

The Democratic National Convention will be held in Philadelphia from July 25 to 28, with the roughly 50,000 attendees expected to rely on rail and subway to travel around the city.

SEPTA has implemented a modified Saturday schedule with less frequent service, causing delays and crowded conditions for the some 65,000 weekday travelers, though most lines will have more frequent service during rush-hour periods.

But the effect of the regional rail service cuts on convention-goers could be limited, because the convention will take place at the Wells Fargo Center in downtown Philadelphia, which most commuters reach by subway.

A convention spokeswoman said SEPTA had committed to running increased service and additional trains during the convention on the Broad Street line, which runs to the Wells Fargo Center.

Bush said the modified schedule and repairs will continue throughout the summer. There is no estimated completion date.

The Silverliner V cars joined the SEPTA system in 2010. Officials said the suspension systems are still under warranty, and the manufacturer is working with SEPTA to locate materials needed for repairs.

Hyundai Rotem officials did not immediately respond to a request for comment.

Philadelphia's rail problems are not unique among major cities.

New York's Metropolitan Transportation Authority is weighing whether it will completely shut down a busy New York City subway tunnel that connects Brooklyn and Manhattan in 2019 for repairs following Hurricane Sandy flooding. In Washington, D.C., the cash-strapped subway system abruptly shut down for a day in March for emergency safety checks and will undergo a year-long rebuilding project that is expected to shut down lines for days.

(Reporting by Gina Cherelus; Editing by Joseph Ax and Leslie Adler)