Two Metro subway trains developed serious ailments during yesterday morning's rush hour, forcing thousands of passengers to be late for work in the two hours it took to cure the problems.

At one moment during the scramble "we had trains sitting at every platform from Rosslyn to Stadium-Armory," subway superintendent Joe Taylor said.

The two trains selected the tracks near the Potomac Avenue station to break down. With both lines blocked, it was impossible to route working trains around the disabled ones.

The series of incidents started at 8:35 a.m. when a Blue Line train heading into the District braked to an emergency stop just short of the Stadium-Armory station. Metro's computer-operated trains react that way when any of the system's many electronic sensors says something is wrong. The train operator was able to persuade his machine to move again, but the train repeated the brake lock-up 8:47 near the Potomac Avenue station.

Five other trains were backed up between crossover switches behind the stricken train, three of them between stations. Passengers on those trains sat and waited, while those in the stations were told first to unload, then later to climb aboard again. In all, seven trains full of people were unloaded at various stations on the Orange and Blue lines.

Metro began to route trains around the stricken train, but one of the rerouted trains (traveling on the wrong track) braked to an unscheduled stop just as it tried to leave Potomac Avenue. Three cars were left at the platform, the rest in the tunnel. Both tracks were blocked.

The first train was finally moved and service of sorts was restored in both directions, but the railroad did not return to normal two-track operations until 10:35. In the meantime, Metro turned back two trains at Federal Center SW, one at Smithsonian and one at McPherson Square. Another train was unloaded at Eastern Market and sent to push the second disabled train back into the Potomac Avenue station.

Metro officials estimated that service to the new Addison Road Blue Line station was interrupted for as much as an hour and that passengers had to wait at the New Carrollton terminal on the Orange Line for about 30 minutes. Erratic service was maintained through downtown and in Virginia.

One of the trains halted because a collector shoe -- the device that picks up electricity from the third rail -- had been knocked off, probably by an object along the track, Metro officials said. They did not know late yesterday what had halted the other train.