"OK, folks, you're finally on a train that won't break down." the operator announced triumphantly as his metro subway pulled out of L'Enfant plaza with a load of delayed rush-hour commuters Wednesday.

The train moved halfway into the tunnel and died. Many of the passengers gigled. They had just been removed from one bad train, then endured the wait for another.

Other passengers were less amused.

"It's getting to the point where a subway breakdown is an excuse for being late." one said.

The problem, once again, was that those gleaming, comfortable, $300,000 Rohr Industries subway cars sometimes refuse to move when they are full of standees. Standing loads are a normal rush-hour occurence on any transit system.

Metro officials indicate privately that they are worried that they have a lemon and that it will be hard to get it fixed. Rohr Industries is going out of the transit-car business and it's last job is completion of the Metro contract for 300 cars.

"The problem is that we don't know how interested Rohr is in making sure these things work." one Metro maintenance expert said. "They're getting out of the business. They're losing money. Their good rail people have already found jobs somewhere else. We might win lawsuit against them 10 years from now, but meanwhile we've got to run a railroad."

Rohr's man in Washinton, George Prytula, said yesterday that "I know of nothing that would cause us not to fulfill our contract . . . as long as we're in business."

Rohr says that it has lost $45.8 million on the Metro cars and has filed a claim against Metro for $48.3 million. Rohr officials have declined to be interviewed on the specifics of the claim.

Metro points to a penalty clause in the contract that syas Rohr must pay $50 every day for every car that is late. All were due in May 1976: about 180 have been accepted by Metro, others are either awaiting shipment or are under construction in the Rohr plant at Winder, Ga. The penalty could total millions of dollars.

It is in that litigious atmosphere that Metro maintenance personnel are trying to solve quickly the most obvious problem: the doors won't close with a standing load and the train won't run if the doors are open.

Erich Vogel, Metro's rail maintenance chief, said there are other problems - a faulty safety device that indicates the brakes are bad when they are good, air conditioning failures, some motors that are overloading and tripping out."But the doors." Vogel said, "are unique, because there are so many of them." Every car has six double doors.

THis scene is becoming a rush-hour regular:

A train shows up late, frequently because it had a door problem at a preceding station. Because it is late, a huge crowd has gathered on platform.

The doors open. Everybody crushes on. The door close. Nothing. They open and close two or three or four times. With luck,the train then moves. But if it doesn't the train operator has to ask people to get off. Meanwhile, other trains are backing up in the tunnel behind the platform.

"Get that bum outta' there," Metro rail chief Anthony Stefanac, one of the subway's imported New York experts, orders his people at Metro control. One the TV-like display, five blips that denote trains are stacked up behind the "bum."

"Unload your train," central orders the operator. "Tell those people there are five trains behind them to pick them up." The train is unloaded; once empty, automatic safety locks can be cut out, the track cleared and the train moved to a siding.

A stalled train sometimes forces Metro to detour other trains around it. That means that a stadium-bound train might show up in a station on the National Airport track. Although announcements are supposed to be made in the stations when that happens, the practice is confusing to commuters.

It only takes one such incident to disrupt a rush hour for 15 or 20 minutes. Last Friday, Metro had four such incidents - four "bums" that had to be removed from service. The result was that 25 scheduled train trips were not made; some people waited 45 minutes for a ride home.

Three of the four bums had sticking doors.

Vogel and his maintenance team are going through every Metro car to readjust the doors, which have already been modified once to deal with the problem. More than 60 cars had been readjusted early this week; Metro wants about 110 good cars to operate the kind of rush hour service it will need to handle the thousands of people who will be forced from buses on Aug. 1 when the Metrobus system is realigned.

Vogel does not guarantee that the readjustment will solve the problem. "The only valid test is to have a car full of people," he said. "I don't have that many people (for a test)." There are 80 seats per car, and the absolute maximum load is supposed to be 240 people.

The Metro cars were supposed to be light to save energy but also strong enough to carry 240 people. Aviation-type construction techniques - where the shell of the car carries some of the load - were employed, as they have been elsewhere. But when the car is fully loaded, it deflects, and pushes the doors out of alignment. If they are improperly aligned, they will not close.

Part of Rohr's claim against Metro is that the design - light weight and heavy load-carrying ability - presented conflicting demands, according to sources.

When Rohr won the Metro contract in 1972, its fixed-price bid of $91 million was almost 25 per cent less than Metro's estimate. Rohr at the time was trying to expand into the transit business: it had just completed cars for San Francisco's Bay Area Rapid Transit and it subsequently won an Amtrack contract.

It suffered enormous losses in the succeeding years. BART is suing Rohr its cars.

Rohr also manufactures Flexible buses, and Metro has purchased 250 of them recently. They are working beautifully, Metro officials say.