Consolidated Rail Corp. yesterday posted a loss of $205.5 million for April 1 to Dec. 31 last year, its initial nine months in business as a sucessor to bankrupt railroads in the Northeast and Midwest states.

On the eve of its first anniversary, the railroad also threatened to halt rail commuter operations between Washington and Baltimore and in the metropolitan New York and Philaelphia areas.

ConRail chairman Edward G. Jordan said notices will be posted today, stating that the commuter trains will be dropped after 60 days. The Philadelphia-based railroad normally carries 1,000 passenger a day between Union Station and Baltimore on four trains.

Other commuter operations that Conrail said would be stopped include all lines in Northern New Jersey, except for former Erie Lackawanna service, with more than 70,000 daily riders; service in Bucks County, Pa., with 10,000 daily riders; service in Bucks County, Pa., with 10,000 daily riders; in Rhode Island, between Providence and Westerly; and Pennsylvania-Reading seashore service in southern New Jersey.

The decision to discontinue commuter service is not based on ConRail's losses but rather the inability or unwillingness of local transportation authorities - including Maryland's department of transportation - to sign contracts promising to indemnify ConRail against the costs of any catastrophic accidents.

Jordan noted that the $205.5 million loss, of which $139.1 million was rolled up in the final quarter of the year, was "better than anticipated on the basis of the $295 million projection of losses by the U.S. Railway Association for the period.

Revenues for the nine months were $2447 billion and for the final quarter, $802 million, about $28 million below expectations. Jordan said ConRail's volume of freight shipments has continued to lag behind projections - partially because of the unusually cold winter.

U.S. Railway, a government agency that planned ConRail and is charged with monitoring its performance, has forecast that bitter winter weather added $100 million of losses to ConRail's books in January and February, on top of nearly $70 million projected earlier.

In terms of freight volume, ConRail is the nation's largest railroad. It employs nearly 90,000 persons and operates nearly 17,000 miles in 16 states. One year ago today it began business as a private, for-profit company planned by the government. Although ConRail will benefit from more than $2 billion in loan aid, losses are anticipated for at least several years.

Under accounting procedures required by the Interstate Commerce Commission for other railroads. ConRail's loss for the nine months last year was $419 million. Under the law establishing ConRail, however, general accounting rules applied to most companies were mandated, resulting in the lower net loss reported yesterday.

The dirrerence involves "depreciation accounting," under which ConRail auditors capitalized $232.9 million spent to rehabilitate tracks and did not charge them all to the 1976 period.

Jordan said ConRail will install 5 million ties and 1,041 miles of tracks this year, resurface 8,300 miles of track and rebuild nearly 17,500 locomotives and freight cars. Capital expenses will be $640 million for new equipment and tracks, including 175 new diesel engines.

The most important accomplishment last year was "proving that the concept behind ConRail works," he stated.

A spokeswoman for Maryland's transportation department said yesterday that commuters on the Baltimore Washington line will be told in notices today that they should not worry about a halt to their service.

Although a ConRail spokesman said no negotiations with Maryland are in progress, the Maryland spokeswoman said a delegation of state officials will travel to Philadelphia today with a subsidy check coverning the March 27 April 30 period.

"We feel we are still negotiating . . . we are very confident we can come to an agreement," she stated ConRail contended, however, that the issue of indemnification is not negotiable since the federal government has ruled that commuter authorities must agree to cover any liability losses. In metropolitan Philadelphia, all jurisdictions except Bucks County have accepted this ruling.

A legal deadline for reaching new commuter agreements with ConRail expired last Saturday. The Maryland spokeswoman said the state is "loathe to go into an open-ended" agreement to hod ConRail harmless for commuter service accidents and said the state has not studied alternative services if the ConRail trains stop.