Parking has become the top problem for Virginia's planned commuter rail service, as local jurisdictions struggle to find land and money to build stations and lots before the trains roll next October.

With less than a year to go, Fairfax and Stafford counties still haven't chosen all their station sites because they are having trouble finding enough land for parking lots, and Fredericksburg officials still are seeking parking lot sites near their existing station.

Prince William County supervisors yesterday authorized the county staff to spend about $6 million in bonds on four stations and three parking lots. At one of the stations, they also authorized leasing and then purchasing a 600-car parking garage estimated to cost $5 million to $6 million.

The supervisors made the decisons at a time when they said they expect to have to cut programs next year because of the slumping economy and state cutbacks.

"We're taking a risk. We know that . . . . It's a good risk," said Prince William Supervisor William J. Becker (R-Brentsville). The supervisors agreed to consider parking fees to help cover the cost.

First proposed 15 years ago, the $60 million Virginia Railway Express will begin with eight daily round trips -- four between Fredericksburg and the District and four between Manassas and the District.

The rail line is expected to lure 4,200 round-trip commuters from crowded interstates, the equivalent, supporters say, of adding another lane to Interstate 95. The 16 rail stations should have enough parking on opening day, because the 4,700 planned spots outnumber the available seats on the trains, said rail planner Stephen T. Roberts.

However, Roberts said he expects parking to be a problem by the end of the first year. Most of the stations will have extra land for parking expansion, but some are surrounded by neighborhoods. Planners aren't sure which stations will fill up first because an updated study of potential riders won't be done until the end of this year.

Transit planners throughout the country say inadequate parking can severely hurt a commuter rail line, because spread-out neighborhoods, such as those in the outer Virginia suburbs, do not lend themselves well to walking.

"The controlling factor does not seem to be seats but parking," said Robert Shreve, marketing director for the Maryland Rail Commuter Service, which serves the Maryland suburbs. "You can never have enough parking."

Washington area residents have firsthand experience with inadequate parking for mass transit.

Metrorail planners estimate their riders could fill 100,000 parking spots, but the system has space for only one-quarter that number of cars.

Parking at the Vienna Metrorail station, for example, was inadequate the day it opened in 1986. Until a new parking garage opened last month, would-be commuters had to arrive before 7:20 a.m. to get a space.

Virginia Railway Express officials say they don't expect the situation to be nearly as severe for their lots, at least not right away because they are running only eight round trips daily, rather than all-day service.

But MARC's Shreve said the service opened a new 150-space lot at Odenton two months ago, and the lot already is full. The Chicago area's Metra system sells parking permits, said spokesman Chris Knapton.

The effort to leave room for parking expansion along the Fredericksburg line has caused part of the problem with finding station sites near Lorton in southern Fairfax and Stafford's Brooke area, officials said. At sites where expansion will be difficult, Prince William and Fairfax may use shuttle buses.

"We could reduce traffic on county roads {as well as on the interstate} if we could get people to assemble where they live and get on a bus," said Prince William Supervisor Edwin C. King, who heads the railway's operations board.

But officials aren't sure they will have the money to start an extensive shuttle system.

Prince William officials had to scrap plans earlier this year to start the county's first internal bus system. Any shuttle bus would have to be taken from the existing commuter bus system, said John Scofield, assistant to the county executive.

Transit planners also warn that commuters may not want to use the trains if they have to ride a shuttle.

"A feeder bus doesn't work everywhere," said Metra's Knapton. "If a bus pulls up at an apartment building and 35 people get on, that's great. But if it goes into neighborhoods . . . and takes 20 minutes and you know it's a five-minute drive, you'll take your car."