Secretary of Transportation Brock Adams and key members of the Senate have reached a compromise agreement on mass transportation funding that will permit development of $5.7 billion in new projects over the next five years.

A Department of Transportation official emphasized last night that the administration has not yet "endorsed" the Senate aid plan but conceded that it is "moving a long ways toward . . . awfully close to" the Adams position. Enactment of such a program this year is uncertain because of a negative attitude toward a mass transit bill to date among House leaders.

In Capitol Hill testimony Feb. 25, Adams had suggested that no new program was necessary at this time to finance additional mass transit programs ant that adequate funding was available to continue already-approved projects through 1979.

Public transport proponents and senators from urban areas immediately expressed concern about the Adams statement, which they saw as an indication that cities with planned projects would find no available federal aid from the Carter administration.

Sen. Harrison A. Williams (D-N.J.), who originally proposed legislation calling for $11.4 billion of new mass transit funding over five years, said yesterday the compromise with Adams means that urban transportation programs in large metropolitan areas now will receive increased aid.

"For states like my own, this legislation will mean the influx of millions of dollars to complete long-awaited and badly-needed projects," Williams stated.

Although no specific figures were available yesterday, congressional aides said the compromise will aid metropolitan New York's mass transit program in particular, which has a backlog of new and modernization projects totaling $1.5 billion.

A spokesman for Williams said there is no way of telling how much new aid will go to metropolitan Washington's transportation authority for subways and buses.

Specifically, under the compromise, features of the Williams bill have been combined with current obligations levels in the Urban Mass Transportation Assistance Act of 1974.

As detailed in recommendations sent to the Senate Budget Committee from the Committee on Banking, Housing and Urban Affairs, the proposed legislation would:

Pump more than $11.5 billion into the nation's mass transit systems over the five fiscal years of 1978 to 1982, beginning next Oct. 1. Of the total, $5.7 billion would be for new projects and operating assistance and $5.8 billion would be funding for commitments already made by DOT's Urban Mass Transportation Administration.

Establish a formula grant program for operating subsidies based on ridership rather than population. This provision will benefit New York City, which has the biggest mass transit ridership in the U.S., a factor not recognized in formulas based on population alone. Altogether, $335 million of operating subsidies would be provided.

Earmark $30 million a year for three years to help subsidize rail commuter lines affected by last year's creation of Consolidated Rail Corp. to take over most bankrupt rail operations in the Midwest and Northeast.

Williams said yesterday the outlook for passage of the compromise is "fairly good given the level of support from the administration . . . Even though this compromise legislation does not go as far as I hoped it would toward making mass transportation one of our nation's top priorities, I think it will make it far easier for us to begin structuring the efficient and modern transportation system we do desperately need."

Both sides claimed that the compromise was close to their original viewpoints. A Williams aide said that where no new aid existed before, Adams now is supporting a $5.7 billion program. A spokesman for Adams said that where Williams originally proposed $11.4 billion of new projects, the smaller amount represented a significant savings.

As for specific projects designated to receive DOT grants and loans in the future, Adams told Washington Post reporters in a recent interview that he would welcome a congressional role in the selection process. "It's an option they have," Adams asserted.

The mass transit program must be approached in the future differently than under the previous administration, which committed all available funds, Adams said.

The new DOT chief wants to modify the method of committing funds by making money available as projects go forward rather than all at once. Under the Adams plan, as funding for some projects winds down, money would become available for new projects.

In other mass transit developments:

DOT announced a new study of a people-mover system at Morgantown, W. Va., to ng similar systems for Cleveland, Los Angeles, St. Paul and Houston.

Otis Elevator Co., a United Technologies unit, won a $2.6 million DOT contract to study and develop control systems for safe operation of automated people-mover system. (very small vehicles or moving stand-on belts).