Metro is single-tracking around a cracked rail near Bethesda Thursday afternoon -- the system's second cracked rail of the year, coming just a day after the first.

The crack, detected during a routine inspection, and its repair shouldn't hurt the evening commute, Metro spokesman Dan Stessel said.

The crack is on the outbound rail, where trains head toward Shady Grove, and is inside a tunnel. It is small, but Metro crews have to repair it because cracks can be dangerous and lead to derailments.

Rail cracks are frequent this time of year, when the cold temperatures cause metal to contract, Stessel has said.

Metro had 28 cracked rails in 2012 and 49 in 2011. The first in 2013, on Wednesday, was on the Green Line, outside Naylor Road.