Once upon a time, when Washington area residents and Congress decided it would be nice to have a subway here, they designed a grand plan to build one that not only would carry people, but also would make money.

In fact, serious studies by learned consultants showed, the subway would make enough money to pay back almost $1 billion in construction costs, borrowed over 30 years, and to pay the interest on that money during those years.

The consultants were wrong. Nobody seriously thinks the subway will make money. Nobody seriously thinks the subway will make enough money to pay off these bonds.

The federal government is holding the bag and is unhappy about it. Here is how it happened:

Bond analysts would not sell Metro's bonds until there was some guarantee beyond fares that the interest would be paid. In 1971, the federal government provided that guarantee, and the bonds were sold.

If Metro defaults on an interest payment, the federal government must pay the bill. The interest on the bonds is expected to be $1.5 billion over 30 years. That means that the total debt, interest and principal, spread over 30 years will be $2.5 billion.

Some of the local jurisdictions that make up Metro insist the debt is not theirs, but Metro's. They are afraid to contribute interest payments for fear that doing so will establish a precedent that will make them liable for years to come.

Interest on the bonds is due at a New York bank every six months. The annual interest payments rise to a peak of $58 million in 1982. If the principal is to be repaid - and there is no requirement for this to happen - a fund can be established for that purpose in 1983.

Metro has met the interest payments so far by using its own investment income and by persuading Congress in a series of short-term agreements to kick in a substantial federal share while waiting for a permanent solution to the problem to be devised. Congress has been contributing 80 percent and Metro 20 percent, following the formula that now prevails for federal aid to mass transit construction projects generally.

Metro's 20 percent has come from investments Metro makes with unspent construction dollars, a source with a limited life span. So far, none of the local governments has had to pay.

Most local governments favor some kind of permanent federal-local sharing formula, probably 80-20. If the bonds are retired under the 30-year schedule, the 80-20 formula would mean that Metro would make annual payments on a steadily increaing scale topping at $75.8 million in 2012. Local government shares for that peak year, it is estimated, would range from $795,000 for Alexandria to $5.7 million for the District of Columbia.

The federal government is known to favor a plan that would retire some of the bonds early, thus increasing one-year costs but saving money over time. Tough bargaining is ahead.

If Metro defaults, some "stern action" would have to be taken, in the words of one federal official.

One "stern action" that has been considered would be for the federal government to withdraw the money it contributes annually to Metro's, operating costs - about $20 million this fiscal year. That idea was suggested in 1975 by the deputy under secretary of transportation, Theodore C. Lutz. Now he is Metro's general manager.