Metro's trains have reached a high degree of reliability comparable with that of other established public transit systems, but there still are problems with the automated train control system, with doors and with brakes.

Unfortunately, the same generally optimistic report cannot be given about the coin-operated parking lot gates at some Metro subway stations. The Metro board yesterday even voted to send a $3 refund to a man who claimed to have been ripped off by the gates.

That collection of facts came in a quarter-inch-thick staff yesterday to the Metro board, which has been asking what it is that goes wrong when things do go wrong on the subway system.

Problems that actually delay trains do not occur frequently, a fact that will be of little solace to an individual who was caught in a recent delay. But the statistics are impressive.

During three 10-day test periods in February, in September and in October, Metro's cars were scheduled to operate for a total of 17,907 hours. In fact, they worked without delay to passengers for a total of 17,858 hours, or a very high 99.7 percent.

During that time, there were a total of 224 occasions when the trains were delayed because of on-board problems. Almost two times out of five, the fault was in the automatic train control equipment.

That equipment is the on-board recipient of a multitude of signals from Metro's master computer. The signals tell the train when to stop, when to go, how fast to go, etc. They a so tell trains where to stop along the platforms and provide other electronic commands to the subway.

If something goes wrong with the computer, the train is instructed to stop, a fail-safe feature that can be overridden only by the train operator with the permission of Metro's central control.

The second big category of problems has been with Metro since opening day. Almost one-fourth of the delays are caused by brake lock-ups that happen because of some mechanical failure in the braking system. The frequency of incidents has been reduced through modifications and valve replacements.

Sticking doors or faulty switches in the door circuits - the most famous Metro problem in the early days of operation - still rank as a problem, but have fallen to third place. Doors are responsible for delays one time in five.

If a door is not closed, or if a switch tells the computer that the door is not closed, the train will not move. When Metro's Blue Line opened in July 1977, crowds of people thronged onto the trains for the first time, the trains sagged (as they are supposed to do) and the doors stuck (as they are not supposed to do). The doors were rehung and other modifications have been made.

The manufacturer of Metro's 300 cars, Rohr Industries, is no longer in the subway business but is providing responsive warranty service and is contributing to technical solutions as the problems arise, according to Erich Vogel, Metro's rail maintenance chief.

"We're still doing a substantial amount of modification as things crop up," Vogel said.

Train reliability has improved along with the experience of both maintenance personnel and the subway motormen, Metro officials said. Problems are discovered and resolved more quickly. Unfortunately, the opening of new lines such as will happen with the New Carrollton extension on Nov. 20 means a number of new operators will be working. That means in turn that train reliability will dip for a few weeks.

Coin-operated parking lot gates were installed at the Fort Totten Metro station at the board's insistence. People must drop a dollar in coins into the hopper - just like on the turnpike - to get out of the lot.

Eric F. Jones of Clifton Park, wrote to Metro that he had lost $3 because he had to go to other gates.

Metro's bureaucrats responded that since he could not prove it, there would be no refund.

The board, after being apprised of the letter yesterday by member Cleatus Barnett, voted to send Jones $3.

Part-time employes have been hired to help clear the parking lots during rush hours and unfoul machines.