Stepping into a Metrorail car as the temperatures soar can bring cooling relief from the hot outdoors -- or it can mean a stifling, hot ride even worse than standing in full sun on a downtown sidewalk.

Metro's rail car air conditioners have been working overtime, but also failing overtime, during the summer's heat wave.

The transit agency says the air conditioning typically has failed on 21 rail cars per day, about 2.5 percent of the daily fleet of about 850 cars during the peak commutes. But on the hottest days, the rates grew even higher, with as many as 66 failures as the temperature climbed to 100 on July 6. That's a bit less than one out every 12 rail cars during rush hour.

"As the temperature increases, naturally it stresses everything that has to do with these systems," Metro Deputy General Manager of Operations Dave Kubicek explained to board members on Thursday.

Hot ride? Metro asks riders who enter rail cars with broken air conditioning to alert the transit system so the car can be pulled from service and repaired. Note the four-digit train number on the inside end doors or outside body of the car, then alert the operator through the intercom in the car. Or call Metro's customer service comment line during business hours: 202-637-1328.

When agency officials charted the number of failures and the daily temperature over a month of hot summer weather, they found a correlation: more failures as the temperatures rose, Kubicek said. The agency tries to repair the rail cars overnight, said interim General Manager Richard Sarles. But when the cooling system fails during daily service, the transit agency will shut off that car and try to get it offline as soon as possible.

Those blacked-out rail cars, though, sometimes cause crowding in the remaining cars, especially during rush hour.

The agency also is trying to reduce the amount of time it opens car doors at outdoor stations, especially at end-of-the-line stops, as the open doors allow cool air to escape.

Meanwhile, the system's oldest rail cars, the Rohr 1000 series that gained attention for crumpling during crashes, are getting full air conditioning upgrades because of problems with the condenser motors, Kubicek said. Twenty-nine percent have been upgraded so far, according to a Metro memo. The agency also is working with the manufacturers of other models to improve their cooling systems.