The first hope for a breakthrough in the 12-day siege of a Dutch train and schoolhouse by South Moluccan terrorists was raised tonight when the government and the gunmen agreed on two mediators in the tense drama in which some 60 persons are still being held hostage.

About 55 of those hostages are being held on a hijacked train ten miles north of here. Justice Ministry spokeswoman Toos Faber said tonight that the first "superficial" contract had been made by field telephone between the mediators and terrorist leaders on the train.

The two mediators, both well-known members of the South Moluccan community in this area, are to meet Saturday - possibly on the train - for what officials hope will be a more extensive discussion.

In recent days, first the gunmen and then the government had rejected each other's suggestions for mediators.

One of the mediators is Ventje Soumokil, the widow of the former leader of the short-lived South Moluccan Republic in the 1950s who was executed by Indonesia in 1966.

The other is Dr.Hassan Dan, 56, a retired general practitioner. He is also a South Moluccan and may know some of the young terrorists, since much of the Moluccan community here has been his patients.

The tiny South Moluccan islands in the Pacific long under Dutch colonial rule, were given up by the Netherlands in 1949 and later came under Indonesian rule. The children of Moluccan nationalists and expatriates who came here then have, in recent years, increasingly ised extremist acts to vent their frustration over the absence of an independent homeland and the precariousness of their own cultural existence.

Faber refused to say what the mediators will discuss and what remain of the terrorists' initial demands.

The Dutch govermment has said it will not allow any hostages to be taken out of the country. Faber also acknowledged that since last weekend the terrorists have not mentioned their original demand that 21 Moluccan prisoners in Dutch jails be fown those prisoners are being held for the train, in which three people were late Amsterdam.

The prisoners' lawyer has said some of them do not want to go with the new group of terrorists.

This has led to speculation that the eight to ten gunmen on the train and the four holding four teachers in the schoolhouse may try to negotiate only their own passage out of the country.

Despite the first steps toward mediation, however, the grim struggle here between the authorities and the terrorists remains very touchy.

In the previous train hijacking it was only after 12 days that the other Moluccans gave themselves up and freed their remaining hostages.

With this siege now 12 days old, an important psychological milestone has been passed and the situation now moves into uncharted dimensions of patience and nerve for both sides.

Dutch officials in recent days have generally said that they expected the current group of gunmen to try to hold out longer than their predecessors.

"There is always some escalation," said Prof. Wybe Van Dijk, the chief of a team of psychiatrists standing by to help ease the mental strain on the hostages when they are released. "Every situation, every new terrorist act, goes a bit longer and gets moreda dangerous."

For the Dutch, who rely on psychological warfare rather than force to free hostages, the lengthening siege piles levels of unpredictability uponthe things they thought they learned from the first train hijacking.

Doctors involved in both episodes feel that the situation evolves in phases;

First, the terrorists suddenly achieve real power when they take over the train , the hostages are at their mercy and the government is stunned.

Then a gradual return to reality takes place, specialists feel, and negotiations begin .

Finally, the terrorists themselves, if the government handles things coolly, begin to feel isolated. They begin considering how it will end and how to give themselves up under the best circumstances.

After the 1975 siege, however, authorities here realized - as other had found in different countries - that under bizzarre conditions of captivity many hostages gradually develop a sense of identity with their captors, both in camaraderie and in ideological terms. Several in the 1975 siege came away, for example, feeling that the Dutch government had not done enough to find a solution to the South Moluccan grievances.

Asked how that could have happened when the gunmen killed three hostages at point blank range, Van Dijk said he "had the impression that stronger danger breeds stronger identification, the feeling that identification with your captor is the only way out."

In the current dilemma, Van Dijk speculates, "I think personal ties will also become stronger and stronger" and there may also be a reverse application, in which the gunmen begin to identify with the hostages.

he explains: "I think they are approaching that phase of feeling identified . . . both the hostages and the gunmen . . . feeling themselves in a dangerous situation. It's sort of a "metoo" feeling. How can we get out of this dangerous situation together."

Van Dijk was asked whether that meant that if stronger personal ties are developing the government has less pressure on it to find a quick settlement.

"That is one way of thinking of it. But if you wait too long you may run into a state of exhaustion, a loss of control!" in which the gunmen "might do strange things."

"Where does one line end and the other begin?" Van Dijk asked. "That's what makes it so important that the man on the field telephone is sensitive and experienced."

The man on the phone to the terrorists throughout this siege and the previous one is Dr.Dick Mulder, the chief psychiatrist for the Justice Ministry.

While the areas around the train of Dutch police and marines, it is the psychiatrists who hold center stage here.