Organizers of Northern Virginia's first commuter rail service are negotiating to buy up to 25 used rail cars from Boston because they are afraid a shipment of new cars won't arrive in time for the planned start in October.

The Virginia Railway Express also needs additional cars to handle more riders. According to new projections, nearly 5,000 daily riders will use the system's two lines, one linking the District and Manassas, the other between the District and Fredericksburg, Rail Manager Thomas R. Waldron said yesterday.

Under a proposed fare plan, a round-trip ticket between Fredericksburg and the District's Union Station would cost $11.90. Monthly passes would reduce the per-trip cost by 30 percent. A D.C.-bound commuter from Fredericksburg would pay $184, or $8.33 daily, for a monthly pass, for example, and a passenger from Woodbridge would pay $140 a month, or $6.37 daily.

Many commuters from Washington's outer suburbs now spend three hours on the road daily, much of that on congested interstates, and are looking to commuter rail to provide a transportation alternative.

"The {new} cars we have ordered come from Brazil . . . and there isn't a great deal of slack in the schedule," said Prince William County Supervisor Edwin C. King (D-Dumfries), who heads the Virginia Railway Express operations board. Buying the Boston cars "will ensure that we have cars to start the operation."

The manufacturer of the new cars, Mitsui/Mafersa, has promised to deliver 20 by October and eight more by the end of November at a cost of $28 million. Waldron said the company's schedule has left little time to correct production errors.

If the Brazilian cars arrive on schedule, the railway would begin service in October with eight trains -- four on each line -- with seats for 2,328 people. The cars to be delivered in November would add 936 seats.

Rail planners have been working from a 1987 study that projected 4,000 people would take the train daily. However, preliminary results from a new study show the number of commuters has increased 25 percent, and many of the new commuters live in Fredericksburg and in Prince William and Stafford counties.

Railway officials said yesterday that the Boston cars, which belong to the Massachusetts Bay Transit Authority, would need minor repairs, such as repainting and maintenance. Each car would cost $40,000 plus repairs, Waldron said, because Virginia Railway would assume the federal transportation grant that paid 80 percent of the cars' initial cost.

Meanwhile, several of the 18 planned stations have fallen behind schedule.

Fairfax County planners told the County Board last week that the Lorton/Pohick station probably won't open until mid-1992, Supervisor Sharon Bulova (D-Annandale) said.

Railway officials are having trouble finding sites at Lorton and at Stafford's Brooke station that would provide adequate roads and cause minimal environmental damage, Waldron said. Still, he said, Stafford officials hope to open the Brooke station by October. Spotsylvania officials are still debating whether to build a station southeast of Fredericksburg.

Tentative fares proposed for the system won't be approved until next year, according to railway officials, when another ridership survey is completed.

The proposal divides the track into seven-mile zones, with passengers paying $3.15 to go one zone, and 35 cents for every additional zone.