The purported solution to one of Metro's most irritating occurrences - the jamming exit gate that refuses to accept your Farecard and let you out of the subway station - is being developed behind a door bearing the sign: Rattlesnake Area, Stay Aware.

Snakes in this desert region are not the only problem for Cubic Western Data, the company that builds and services Farecard equipment for the Washington Metro.

Cubic Western is getting bad press because Farecard does not always work, either in Washington or in San Francisco - the only two cities that use such complex fare-collecting equipment for their rapid transit systems.

"We're not satisfied with the reliability either," said R.L. deKozan, Cubic's vice president and director of marketing. "But let's talk about reliability. We have 800 machines in operation in Washington and we had 562 failures in January. That's .7 of a failure per piece of equipment per month. The specifications call for one failure per machine per month. We're within the specs."

Failures are defined by Cubic Western as problems that require a technician to fix. The Metro user has another definition: if the machine does not work, it's a failure, regardless of whether it can be fixed by a technician, a station attendant or a swift kick.

DeKozan sighed. "Even though we're within the specs, we do not consider it good enough."

In a day-long tour of Cubic's plant and facilities, deKozan and other specialists discussed what they see as major problems with the Farecard system, what they are doing about it, and what they expect Metro to pay for in addition to the $52 million in the basic contract.

Every regular subway rider has encountered three of the four main problems. The fourth drives Metro's financial planners batty, but is out of public view. The four problems:

The ticket transport jams. The transport is a collection of pulleys and belts that grabs your Farecard and carries it through the entrance of exit gates before returning it to you. Similar transports are also located in the Farecard vending machines and the Addfare machines.

The coin acceptor in both the vending and Addfare machines jams, then accepts no coins at all and refuses to return the ones you gave it.

The bill validator on both the Farecard vending and Addfare machines sometines refuses to accept perfectly good $1 and $5 bills as it is supposed to.

The Farecard people-counting equipment continues to lose thousands of people every day, without a trace. According to total numbers taken from the "in" and "out" counters on each gate, about 4,000 people more per day enter the Metro system than leave it. Metro's financial planners, being conservative, use the lower number when they report rider statistics.

Similar problems exist with the Cubic Western equipment on San Francisco's Bay Area Rapid Transit (BART) system, specialists there told a reporter during an interview last week, although BART does not have disappearing riders.

The transports will be fixed soon, Cubic program manager Thomas B. Tuttle promised. A new transport has been developed and installed on gates at the critical Farragut West station.

The new transport will accept wet cards, bent cards, sticky cards and cards with corners missing - at least in the laboratory as demonstrated for a reporter. Metro riders undeniably bend their Farecards, stick their Farecards in sweaty pockets and pick their teeth with their Farecards then wonder why the machine will not accept them. With the new transport, it should.

If tests at Silver Spring - an outdoor station - are as successful as those at Farragut West, an underground station, the modification will be made on all equipment by May 15, Tuttle said.

The new transports will be installed at Cubic Western's expense, Tuttle and deKozen said.

That is not true for new coin acceptors and bill validators, both of which are being developed by Cubic Western.

The present coin and bill-handling equipment is purchased from an outside supplier, but has not worked adequately under the stress of transit use, deKozen said. "The average Farecard vendor processes 330 bills and 900 coins a day," he said.

A new coin acceptor will probably be ready for installation in Metro machines this fall. It works on an electrical instead of a mechanical principle and has fewer moving parts. it will cost $200 per machine.

A new bill validator is at least a year away, deKozen said.

The final total cost to Metro will have to be negotiated.

That leaves the problem of the disappearing people. Neither Cubic nor Metro has an explanation, but Cubic's Tuttle thinks some combination of three things contributes to the difference between the number of people counted as entering and the lower number counted as leaving:

During rush hour, when stations are crowded, Metro station attendants let passengers bypass the exit gates if the passenger's Farecard has only the value of the exact fare remaining. Attendants are supposed to feed the cards back into the system. If they do not, there would be more people "in" than "out."

There is a method of cheating the Farecard system that could result in one person being counted twice on entry but only once on exit.

The computer in the entry gates is receiving a change in instructions because, under some limited circumstances, it counts a person more than once.

Both BART and Metro officials said that Cubic equipment has been improving as time passes. Both also said that the equipment fails most often during rush hours, when thousands of people are using the systems.

During a recent Washington evening rush hour, Nicholas Roll, Metro's assistant general manager for transit services, hustled to the busy Metro Center station to assist a crowd of people waiting to buy Farecards. Six of the eight vending machines said "Out of Order" in belligerent red letters. About 20 people stood in line at each of the other two machines.

Roll dispensed pre-encoded $1 Farecards and stuffed the bills in his pocket. It improved the dispositions of those standing in line, but did nothing for Roll's.

"This has got to get better," he muttered. "If you ask them one question, ask them why it slways breaks down in rush hours."

DeKozen responded: "If there's a weakness, it's going to show under stress. Our equipment gets 9.6 million transactions a month, and there is only one failure for every 15,000 transactions."

Cubic Western is a subsidiary of the Cubic Corporation and in 1977 accounted for 41 per cent of the parent corporation's $50 million in sales in industrial products, according to the annual report.

The contract to build Washington's equipment is Cubic's biggest. The company has also built fare equipment of various descriptions for rail systems in the Chicago and Philadelphia areas. It now has contracts from the Metropolitan Atlanta Rapid Transit Authority (MARTA), from Hong Kong, and from Sydney.

"Basically, Metro has a good (fare-collecting) system," said deKozen. "We're applying ourselves to getting the bugs out. We will. If we don't, we will just have to be held accountable."