More time and additional money will be needed to complete a modernization of railroad passenger service along the densely populated Northeast Corrider between Washington and Boston, the Carter administration told Congress yesterday.

Secretary of Transportation Brock Adams said the rehabilitation cannot be completed before the end of 1983 despite a congressional mandated that work be finished two years earlier.

In addition, the project new will cost an additional half-billion dollars, which Congress must approve.

In a 1976 railroad revitalization act, Congres called for a massive improvement in Washington-Boston passenger service by the end of 1981, including a schedule of 2 hours and 40 minutes for Metrolimer service between Union Station and New York and 3 hours and 40 minutes from Now York to Boston, compared with current schedules of 3 hours and 20 minutes and 4 hours and 50 minutes, respectively.

Metroliner service was inaugurated at the start of this decade by the old Pwnn Central Railroad, just before the rail firm plunged into bankruptcy. Amtrak, the federally subsidized intercity rail passenger firm, subsequently rail passenger firm, subsequently took over the corridor operation but on-time service has been deteriorating badly because of equipment and track problems.

Since the Northeast appeared to offer the greatest potentail for operating passenger service without singificant deficits -- compared with other Amtrak routes -- Congress and the Ford administration moved to rectify operating problems along the line and bring trrue high-speed service to the region through the 1976 legislation.

But a "redirection study" sent to Capitol Hill yesterday declared that the $1.75 billion authorized for corridor work "will not buy" what was mandated. "Overoptimism, understimation, inflation and the immense problems associated with creating and managing" the overall project required a new look at the costs and schedule, according to the Department of Transportation report.

Adams told reports that the administration now is recommending a $2.5 billion program, which would add $654 million of new funds -- $66 million from public grade-crossing money, $30 million to rehabilitate Union Station here and the balance to come from new authorizations.

Because the new funds would come in later years and not represent increases in annual spending levels, the more expensive project remains "within President Carter's guidelines" for reduced federal spending, and has White House blessing, Adams declared.

If Congress approves the "redirection" program, which is expected, a new fleet of 67 locomotives would be purchased and 34 Metroliners would be refurbished. In addition, some Washington-New York trains would begin operating with shorter schedules before the whole project is completed.

Under the new proposal, all passenger service between Boston and Washington would be operated behind electric locomotives (the Boston-New Haven segment is not electrified now). Freight trains and commuter lines that use the Amtrak route also would be electrified.

Amtrak is forecasting an increases of 18 percent in trains per day along the corridor between now and the mid-1980s-possibly including express trains between Union Station and New York with 2-hour, 30-minute timetables.

Adams emphasized yesterday that the administration continues to support a proposal made last year that would return Union Station primarily to its original purpose as a railroad terminal, plus associated facilities for bus services.

A congressional impasse over Union Station -- converted to a national visitors center for the Bicentennial but behind schedule and at more coast than planned -- prevented any decision last year. The House Public Works Committee wanted to kep trains out of the center and finish a parking garage and road facities at a cost of $36 million while the Senate supported a more expensive $52 million expansion of rail facilites and the addition of an intercity bus terminal.

Aleady, the federal government has spent some $46 million for the facility as it is today -- ka largely unused central visitors complex and a small rail terminal to the rear of the building.