After more than four years of delay, the way has finally been cleared for construction of the John F. Kennedy Memorial Library complex in Cambridge.

Officials of the Massachusetts Bay Transportation Authority (MBTA) and the Penn-Central Railroad yesterday signed an agreement which will enable the MBTA to vacate its Bennett-Eliot car barns-the site of the library complex. The 12-acre site lies across Boylston St, from Eliot House.

Under the agreement, the MBTA will pay the Penn Central $7 million for a tract of land near South Station in Boston, and will relocate the car barns there.

Two Years to Go

An MBTA spokesman said yesterday that the car barns should be moved completely out of the library site within two years. The long-awaited construction of portions of the library complex may begin even sooner. depending on the exact plans for the complex, and on which portions of the yards the MBTA vacates first, said I. M. Pei, the New York architect who is designing the library.

"It's certainly very good news," Pei said of the agreement, which ends years of frustrating attempts to relocate the car barns. The attempts began even before the Commonwealth of Massachusetts gave the Bennett-Eliot site to the library trustees in January of 1966, with the condition that the MBTA find a new site for its car barns before any work begins on the library construction.

Hard Luck

The MBTA successively tried to relocate the barns to sites in Dorchester, Mattapan, and South Braintree, but residents of each community-feeling that the barns would be a nuisance-blocked the attempts through legislative action or in the courts. The negotiations which resulted in yesterday's agreement began last spring.

The site near South Station where the car barns will be put has long been a railroad switching facility for the Penn Central. The railroad now has one year to relocate that facility to Reading or Allston.

The delays over clearing the library site mean. however, that Kennedy Library officials are now faced with a difficult decision. During the past years, the estimated cost of building the library has risen by 35 to 50 per cent. The $8.5 million already collected for the library construction will not be enough to pay for the facilities originally planned.

"We will have to decide whether we should reduce the program or look for more money. "It will have to be one or the other, or perhaps some of each," Pei said.

The Trustees of the library, and the Building Committee-composed of representatives of the library. Harvard, and the Federal Government-will soon convene and begin deciding what to do, Pei said.

Due to the relocation delays. he said, "we really haven't advanced very far inplanning since late 1964. At that time we completed a conceptual proposal for the area."

As tentatively envisioned, the 12-acre site would be divided up between several different facilities:

Harvard's John F. Kennedy School of Government-now located in Littauer Hall-would occupy two acres. The University must itself raise money to build the government school facilities which were not part of the Library's fund raising effort. Some $10 million has, however, already been collected to endow the Institute of Polities, which is a part of the school.

The City of Cambridge would receive two acres for access roads, and perhaps some parking facilities. "This, we feel, is a development that must not be allowed to upset the [traffic] and should help it if possible," Pei said.

The Commonwealth of Massachusetts. in recognition of its gift of the site, would receive several acres, which would he used as a memorial park.

The remainder of the site would be divided between the Library itself and a number of "related facilities," such as meeting places for youth groups.

Over 15 million pages of documents from the Kennedy Administration as well as some 10,000 items of memorabilia-including the President's favorite rocking chair-have already been collected for the library. The items are presently located at the Federal Records Center in Waltham, Mass., where a staff of four men is sorting them out.