A Suffolk Superior Court judge yesterday effectively cleared--for the time being--the last legal obstacle holding up construction of the John F. Kennedy Memorial Library.

In dismissing the Town of Milton's suit to block the extension of MBTA rapid transit lines from Ashmont to Mattapan, Judge Frank W. Tomasello apparently ended a four-year delay in construction of the library.

Ground-breaking on the library has been held up because the MBTA has not been able to find a site to move its repair yard--now located on Bennett St. opposite Eliot House--to make way for the library.

The latest solution proposed extension of the present Harvard-Ashmont rapid transit line to Mattapan and construction of the yards there. But the Town of Milton, through which the new line would travel, objected and filed the suit that was overturned yesterday.

Construction on the library could begin soon--unless Milton decides to appeal Judge Tomasello's ruling.

In his decision, Tomasello ruled that the MBTA compiled with all legal requirements in connection with the $12.8 million Mattapan project. Milton had argued that the extension was not included in the original mass transportation plan approved for the MBTA.

Tomasello, however, said the plan was "duly approved by the Advisory Board of the MBTA on February 29, 1968." He also said that the Authority had properly consulted the Commerce Department and the Metropolitan Planning Council.