An epidemic of locked brakes, jammed doors and other mechanical failures put 13 of Metrorail's 43 rush-hour trains out of action yesterday morning, stranding tens of thousands of irate commuters on crowded station platforms. It was one of the subway's worst rush-hour performances in memory.

Monday night's light snow and temperatures in the teens were tentatively blamed by Metro for many of the breakdowns. Ever since the first subway segment opened in 1976, subway cars have tended to falter when temperatures drop too low.

The weather took a toll on Metrobuses, too. Of 1,530 buses in the rush-hour fleet, 83 broke down, though officials said that was a good showing, given the cold. In most breakdowns, the problem was familiar--frozen air lines, which affect brakes, doors and the "bellows" suspensions the buses ride on.

But the worst delays occurred in the rail system. For many of the estimated 140,000 people who ride the subway daily, the normally routine trip to work became a trial, with 10-minute trips stretching into an hour and more.

Crowds built up on station platforms around the system, as voices over loudspeakers gave what often turned out to be false assurances that the waiting periods would be short. People stood shoulder-to-shoulder, then jostled one another to try to board trains that did arrive--and often cars were already full from previous stations.

Six of the 13 trains that failed between 4:52 a.m. and 9:08 a.m. yesterday apparently had brake problems, a Metro official said.

Maintenance chief Erich Vogel said that in many of the crippled trains, fuses protecting individual cars' "auxiliary systems"--hydraulics, air conditioning, compressed air, for instance--had blown. Vogel suggested that moisture left by melted snow may have caused the fuses to blow.

When that happens, a car's brakes automatically stop the train or prevent it from starting. Operators normally shut off the brakes in the one car and then move the entire train to a maintenance yard, using the other cars' brakes on the way.

On two trains, the doors failed to operate. Such failures are often caused by dirt or anti-icing chemicals getting into the grooves. The remaining breakdowns were apparently caused by problems with propulsion, switches or circuit breakers.

The widespread failures occurred despite Metro's attempts to insulate its fleet against the cold by parking the trains for the night in tunnels. They began at 4:52 a.m., when operators found the brakes locked on a train that had been stored underground at the Benning Road station.

After that, the rail control room at 600 Fifth St. NW was besieged with calls from other train operators: At 5:24 and 5:25, two trains at National Airport had reported failures; 20 minutes later a train at New Carrollton had reported propulsion malfunctions, and so on until 9:08, when an operator at Rosslyn reported that his train, too, had problems with power.

Altogether, 13 trains were knocked out. However, Metro officials said that some were back in service before the end of morning rush hour.