PHILADELPHIA (CBS) — There no good time for a subway derailment. But a jam-packed rush hour is the worst.

Twenty-five years ago this week, Federal investigators were trying to figure out how and why a Market-Frankford El train jumped the tracks near 30th street.

KYW reporters who raced to the scene found chaos. When the smoke cleared they reported that four people had died and as many as 160 had been injured.

They had to be removed from the rail tunnel and carried to ambulances jamming Market Street.

It was the deadliest accident in Septa history. The focus of the probe turned to a motor that had come loose under one of the cars, questionable inspections and a motorman who tested positive for drugs.