A cracked rail slowed down Metro's Red Line for about five hours on Thursday, in the 26th case this year of a broken rail on the transit system, according to Metro.

Metro has been dealing with dozens of these cracks in recent years. Last year, the agency had 49 cases, and in 2010, 33, while 2009 had 19 and 2008 had just seven cases. This year the numbers appear to be tempering, though still presenting challenges for the agency -- and its riders.

The latest crack occurred near the Bethesda station, causing trains to share a single track for hours as crews repaired the opposite tracks. But because it occurred between the morning and evening peaks, it didn't have as much impact as some recent incidents.

Cracked rails
2008 7
2009 19
2010 33
2011 49
2012 26*
* Through Friday

In July, a Green Line train derailed when the rail buckled in what is known as a heat kink. And two cracked rails on different lines on one day in January caused massive delays across the system.

No one has been seriously hurt in the recent cases, but they pose major safety hazards and hassles.

It's not clear what is causing so many cracks to occur. At times, Metro has blamed fluctuations in temperatures. But Thursday's incident, as with about half of them in recent years, occurred underground, where temperature fluctuations are not as extreme.

Earlier this year, Metro touted a new piece of equipment called a track geometry vehicle as a key tool in helping resolve the problem. The machine uses thermal imaging and lasers to give a better picture of the tracks than the contracted ultrasonic equipment the agency currently uses several times each year to examine the tracks. It's the difference between an X-ray and a three-dimensional MRI and can pinpoint trouble spots before full cracks form. The agency has said it plans to run the track geometry vehicle monthly. Metro employees also walk the tracks to visually inspect them.

The new machine has been a long-promised panacea. The transit agency was supposed to receive the vehicle in 2010, according to a 2008 Washington Post article. Metro spokesman Philip Stewart has said the agency plans to begin using the machine this month, after receiving it in July, following testing.