Unfortunately, there is often an unrealistic air of expectation whenever U.S. and Soviet leaders gather in one of their summits. That's already true of the President Ronald Reagan, President-elect George Bush, and Soviet President Mikhail Gorbachev meeting in New York this Wednesday, even though it's only billed as a brief, informal luncheon session.

The encounter is not designed to allow any serious negotiation or to produce any serious agreements. There is not even a formal agenda, although some weighty matters are sure to be discussed in the few hours the trio is together.The whole session is really more of farewell for Reagan and a get-acquainted event for Gorbachev and Bush who will be dealing with each other for the next several years. Mostly, it will be an opportunity to size each other up. The public should view it strictly in that light.

As White House spokesman Marlin Fitzwater points out, Reagan will be leaving office in a few weeks and is in no position to make any promises he can't keep. Bush, although the president-elect, has not yet taken office and is not going to make any commitments before he even occupies the White House.

Yet there are signals that Gorbachev views the brief meeting as something more significant than a mere social call. Soviet analysts have said repeatedly that the Kremlin boss has more in mind than just a cordial talk. He wants something more substantial to come from the encounter.

Clearly, Gorbachev feels under some pressure to produce results in his foreign policy, if only to strengthen his hand at home where he is trying to restructure the Soviet bureaucracy, pump economic life into the stagnant Russian economy, and steer the communist state in new directions.

All of that is not easy. There are hardliners in the Kremlin, there is ethnic and political unrest in the provinces, and just how much the Russian bear is really changing is open to question. In any case, the U.S. cannot afford to eagerly leap at whatever bait Gorbachev may dangle in front of the outgoing and incoming American presidents.

Cool restraint is the order of the day. Even if Gorbachev does come up with a blockbuster proposal of some kind, the only U.S. response under the circumstances would be to welcome it, but to simply take it under advisement for futher examination.

However, the luncheon meeting does have some significance, even if the superpower leaders only talk in general terms about some of their concerns. The fact that such meetings can be held in a casual way is a sign of how far U.S.-Soviet relations have come in recent years.

While no one should expect great things out of the New York encounter, the fact that it is taking place at all is grounds for at least some guarded hope that there will be less superpower friction in the years ahead.