The Washington Post » Rust was found on the steel rail that fractured in a Red Line derailment near Farragut North last week, but the cracked rail that probably caused the crash would have been imperceptible to track walkers and ultrasonic inspection technology, Metro officials said Thursday. Agency investigators are awaiting the results of metallurgy testing by outside experts, which are expected next month and may give a clearer indication about what caused the eight-foot section of track to crack at the start of the morning rush on Jan. 15.