Photo by PoPville flickr user ep_jhu

From WMATA:

Metro may slow trains to 35 mph in above ground sections of the system under certain extreme weather conditions, agency officials announced today.

The decision to implement “slow orders” for portions of the rail system will be based on several criteria, including the actual temperature of the rail. Metro track inspectors routinely measure rail temperature using a number of devices including infrared thermal “guns” and direct contact gauges. The combination of direct sunlight and high ambient temperature can cause rail temperatures to reach 135 degrees or higher. On particularly hot days, Metro implements a regimen of enhanced track inspections, with additional inspectors and supervisors walking and riding the system for signs of heat-related issues.

Under the new practice, if inspectors detect rail temperature higher than 135 degrees on a section of rail, a slow order will be placed into effect for that rail line. However, Metro officials may implement slow orders using other, more conservative criteria, including an average median temperature, during periods when the region is experiencing a prolonged (3-day) period of heat with insufficient nighttime cooling.

“While these speed restrictions may cause some delays and inconvenience, they are intended to put the safety of our passengers first,” said Dave Kubicek, Metro’s Deputy General Manager for Operations.

A heat kink can occur due to the expansion of steel rails during extremely high temperatures. If the force of an expanding rail cannot be constrained by the ties, clips or ballast, a sudden release of pressure can cause the rail to move laterally, resulting in a “kink.”

The speed restrictions are intended to improve a train’s ability to stop in the event that a heat kink develops on the rails.

Update 3PM:

“Metro has imposed a 35 mph speed restriction on aboveground portions of track on all lines as of 2:30 p.m. Tuesday. The speed restriction is expected to remain in effect through this evening’s rush hour.

Customers should expect longer travel times as trains operate slower than normal in aboveground territory.”