Negotiations aimed at heading off a nationwide railroad strike broke down last night, and unions announced strike actions beginning early today that likely will halt a third of the country's freight traffic and inconvenience thousands of passengers.

Unions representing 235,000 freight rail workers kept exact plans secret last night, but it appeared that most picket lines would go up at 7 a.m., just an hour before a House committee meets to begin drafting legislation to order the strikers back to work.

A nationwide rail shutdown is expected to have an immediate impact on the shipment of approximately one-third of the nation's goods and lead to layoffs for more than a half-million workers.

Transportation Secretary Samuel K. Skinner, echoing earlier statements by President Bush, warned that a railroad strike could seriously damage hopes of economic recovery. "We've got a very, very precarious economy that's just on its way coming back. We cannot afford to have that recovery interrupted by an unnecessary strike," Skinner said.

A union spokesman refused to say whether the strike will be selective, but railroads have made it clear they will shut down completely if pickets go up at key terminals.

Amtrak passengers traveling between Chicago and the East Coast were the first to feel the strike, even before today's 12:01 a.m. EDT deadline. Conrail, a freight railroad that operates several trains for Amtrak, ordered passengers transferred from several trains to buses in the middle of their trips, saying they could not guarantee union crews would not abandon freight trains on the main line.

Unions said, however, that any shutdown would be orderly, with crews taking trains to the next crew-change point or terminal. A railroad management spokesman said no strike activity was noted in the early hours after the deadline.

The effect on passengers is likely to be spotty. Amtrak's lucrative Boston-Washington Northeast Corridor is not directly involved in the strike, nor are commuter operations in New York, Philadelphia and Boston. A judge ordered continued commuter train operation in Chicago. However, other Amtrak routes and several commuter operations are expected to be affected, including an estimated 9,000 MARC commuters in the Washington metropolitan area.

While passenger troubles always draw public attention, it is a shutdown of freight operations that is bringing congressional action. A number of industries may shut down or curtail operations within days, including automobile assembly and some chemical production.

Chrysler Corp. said yesterday it only has three days' worth of parts in its assembly plants inventory, indicating it would have to shut down if a longer strike cuts its supplies. This problem faces nearly all major goods manufacturers who have converted to the Japanese "just-in-time" low inventory manufacturing approach since the last nationwide rail strike in 1982. That strike lasted four days.

Rep. John D. Dingell (D-Mich.), chairman of the Energy and Commerce Committee, called a hearing for 8 a.m. today and said a strike would be "dealt with as expeditiously as possible," but served notice that Congress, not the White House, would draft any strike-ending legislation.

Eleven unions representing employees of the nation's major freight railroads became free to strike at midnight when a 30-day federal "cooling off" period expired. The government exhausted all legal avenues for ending a strike under federal labor law and now must seek special legislation from Congress if it wants to intervene.

Sen. Edward M. Kennedy (D-Mass.), chairman of the Labor and Human Resources Committee, said Congress was prepared to begin moving immediately on legislation. But he said he opposed a White House suggestion that Congress simply enact the recommendations of a special presidential emergency board named last spring to try to settle the issue.

Kennedy said some of the board's 120 pages of recommendations are "extremely vague," and indicated he felt there should be "further winnowing down of differences."

Dingell signaled yesterday that he too may favor asking the emergency board members either to make further recommendations or to appoint other panels to deal with the knotty and seemingly deathless issue of union work rules. The idea of a second panel is being "very actively considered," Dingell said.

Kennedy, asked if he anticipated differences on strike-ending legislation between Congress and the White House, said he believed an agreement could be worked out with Skinner, but he appeared less confident about others in the administration. Kennedy said it was "possible" that Congress could act on strike-ending legislation today, but added, "I don't think you could say with absolute certainty."

Kennedy and Dingell met yesterday with Senate Majority Leader George J. Mitchell (D-Maine) and House Speaker Thomas S. Foley (D-Wash.).

Complicating the legislative picture is the fact that three of the 11 unions -- including the powerful Transportation Communications International -- have reached tentative agreement with the railroads along lines suggested by the emergency board, effectively setting a contract pattern for health coverage and wages.

But those unions did not face the issues of work rules confronting the United Transportation Union (UTU) and the Brotherhood of Locomotive Engineers, both of which represent the train crews. The issues are basic: How many people does it take to safely operate a train, and how many miles must a crew travel to earn a day's pay?

The emergency board recommended the UTU renegotiate all its agreements on crew size, an action the union estimates would cost it 30 percent of its membership. Larry McFather, president of the Brotherhood of Locomotive Engineers, said yesterday his union had no choice but to strike. "Our people have been cut to the bone," he said.

The emergency board recommended that railroad workers get a lump sum payment of $2,000 when the contract is signed -- presumably to make up for the wage freeze in effect since July 1988 when the old contract expired -- and a 3 percent general wage increase beginning July 1 this year. A series of lump sum payments would follow over the next several years, along with another 3 percent general wage increase in July 1993 and a 4 percent increase in 1994.

Staff writer Helen Dewar contributed to this report.