PHILADELPHIA (WPVI) -- The National Transportation Safety Board issued a stern warning to SEPTA on Wednesday, urging the transit agency to take immediate action due to fire risks in its fleet of Silverliner IV railcars.

The warning follows a series of fires this year on the following dates:

Feb. 6, in Ridley Park

June 3 in Levittown , Pennsylvania

July 22 in Paoli , Pennsylvania

Sept. 23 in Fort Washington, Pennsylvania

Sept. 25 in Philadelphia

The NTSB said the outdated design of the railcars, combined with SEPTA's maintenance practices, "represents an immediate and unacceptable safety risk."

Investigators found that the safety issues cannot be fully addressed without a comprehensive retrofit or replacement of the fleet.

"The NTSB also found that SEPTA's current operati

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