HARRISBURG, Pa. -- Federal transportation safety officials told Philadelphia's mass transit agency this week that it should shelve an aging electric railcar model that is heavily used in its regional rail fleet until it figures out how to stop them from catching fire.

The recommendation from the National Transportation Safety Board came after it investigated five fires this year involving the Silverliner IV passenger railcars used by the Southeastern Pennsylvania Transportation Authority, or SEPTA.

In addition to suspending operation of the Silverliner IV fleet until it can fix the cause, the agency said SEPTA should fast-track the replacement of the Silverliner IV fleet or retrofit cars to meet modern fire safety standards and add systems to give the train crew detailed information on when dynamic brakes or other electrical systems aren't functioning normally.

All five fires forced everyone aboard to evacuate -- in one case, as many as 350 passengers -- with a few minor injuries reported. One railcar was involved in two of the fires, and two other railcars were destroyed, the NTSB said.

SEPTA is one of the nation's largest mass transit agencies, carrying 800,000 daily riders on buses, trolleys and rail.

The recommendation comes at a time when SEPTA and major transit agencies around the U.S. are fighting for more public funding as they struggle with rising costs and lagging ridership.

In its report, the NTSB was critical of SEPTA's maintenance and operating practices.

That, combined with the outdated design of the Silverliner IV railcars, "represents an immediate and unacceptable safety risk because of the incidence and severity of electrical fires that can spread to occupied compartments," the NTSB said.

The NTSB traced the fires to different components, including electrical components associated with the train's propulsion system, the dynamic brakes and a traction motor.

SEPTA did not immediately respond to questions about whether it would or could comply with the recommendations.

In its budget report issued earlier this year, SEPTA reported that ballooning material, manufacturing and construction costs has made it more expensive for it to replace the Silverliner IV fleet.

Still, it said the replacements are "long-overdue investments" and "can no longer be delayed."

It put the price tag at nearly $1 billion to replace its 230 Silverliner IV cars built by General Electric in the 1970s.

However, SEPTA also projected that the design, procurement and construction timeline for the replacement would stretch until 2036.

Federal safety board tells Philadelphia's mass transit agency to shelve railcars implicated in fires