Prince William County officials said yesterday they are likely to agree to the creation of a transportation agency empowered to levy a 2 percent tax on gasoline that could help finance an experimental commuter rail line between Fredericksburg and Washington.

The rail project is favored by Edwin C. King, chairman of the County Board of Supervisors, who said the long-awaited service, now bogged down by legal and financial problems, could start in the fall. "This doesn't solve all our problems, but it certainly is a step in the right direction for the entire region," said King, a Democrat from Dumfries.

Creation of the Potomac and Rappahannock Transportation District, a regional authority that also would include Stafford County and the City of Manassas, was to be considered by the Prince William supervisors last night. But after a hearing yesterday, the board agreed 6 to 0 to delay their vote on the issue until July 1. Manassas officials agreed last week to participate in the authority, and the Stafford supervisors were expected to take similar action this week.

King and other Prince William officials hailed the proposed agency as the first major attempt by Northern Virginia's outer suburbs to pursue a regional solution to worsening highway congestion.

"The interstates have expanded the entire Washington region," King said. "It's not unusual now for people to work 50 miles from where they live . . . . We've got to take a broad approach to that problem."

The Potomac and Rappahannock district would be the outer suburbs' counterpart to the Northern Virginia Transportation Commission, which Fairfax County and other localities nearer to Washington created years ago.

But the proposed tax drew strong opposition yesterday from Prince William service station owners at a hearing before the supervisors.

"We're not saying that some form of rapid transit system is not needed," said John B. Vance, president of the Northern Virginia chapter of the Virginia Gasoline and Automotive Repair Association. "We're simply pointing out that it is grossly unfair to 90 percent or more of the citizens of Prince William County who will probably never use the system to make them subsidize the relatively few who can and will use it."

One of the supporters of the proposed district is Fairfax County Supervisor Audrey Moore (D-Annandale), who gave Prince William board members balloons and toy train whistles to commend their "leadership" in backing the new district. Moore, a longtime advocate of commuter rail, said the agency would be crucial in overcoming funding problems for the project.

Fairfax County Board Chairman John F. Herrity, a Republican, agreed, saying the district would help avoid the "Arlingtonization of Fairfax County." The thousands of Fairfax commuters who pour onto Arlington roads each day have long created tension between the counties. "I really applaud this. It is in the best interests of Fairfax County as well as Prince William," said Herrity.

If the rail project is completed, it would carry a total of 3,000 commuters during eight trips daily to and from Union Station. "That's the equivalent of a full lane of traffic on I-95," said Prince William transportation planner John Schofield.

However, some other Prince William officials, including Supervisor Donald E. Kidwell (R-Woodbridge), said they could not support spending money for commuter rail if inexpensive liability insurance for the project is not found. Insurance is seen as the principal hurdle in the way of the project.