Reston express bus commuters yesterday chartered five Metrobuses to get them home from downtown Washington, a final admission that -- at least for the moment -- privately owned Colonial Transit is not capable of providing the service.

Furthermore, The Washington Post has learned, the Interstate Commerce Commission has launched a full-scale investigation into Colonial's service between Prince William County, Fredericksburg and Washington after a deluge of complaints from commuters.

Colonial which carries about 5,500 people from outlying Northern Virginia communities to the Pentagon and Washington every day, recently cut 14 routes from its Prince William County service.

Gary Penn, president of Colonial Transit, confirmed both developments yesterday. Further, he said, 41 of his 147 buses are out of service because of major maintenance problems. A reduction in service, Penn said, was necessary "until we're in a position to get our fleet back.'

Colonial Transit's buses are between 12 and 24 years old.

The Metrobus charters to Reston were negotiated in the past week because of steadily deteriorating service by Colonial to Reston Commuter Bus, (RCB), a volunteer nonprofit corporation of Reston citizens set up to provide commuter service.

Thomas W. Brahms, acting chairman of RCB, said yesterday that Colonial had missed as much as 30 percent of the Reston schedule on some days in January and had been missing 10 percent of the schedule on average.

"We're going to turn Reston Commuter Bus around," said Brahms. "We're getting great cooperation from everybody."

Brahms and Penn both said that a modification in Colonial's contract with RCB had been agreed to so that Metrobuses could be substituted on some Colonial Transit runs.

RCB once contracted with Metro for all of its service, but sought out Colonial Transit in 1974 after Metro's price per bus increased from $40 to $66.91 within a year. The cost per bus from Colonial is $41.41, plus a fuel adjustment charge that is passed through to RCB.

Before RCB could use Colonial, it had to receive regulatory permission from the Washington Metropolitan Area Transit Commission, permission Metro opposed. RCB won, and Metro was fired as the charter operator.

Metro's buses will cost RCB about $100 each -- the standard charter rate. A long-term agreement is being negotiated, Brahms and Metro officials said.

Colonial will continue to provide the majority of the Reston service under the old contract, Penn and Brahms said, but Metrobuses will be chartered to fill the rest of the schedule, RCB has about 40 runs each day to carry 1,300 Reston citizens to work and back.

Ivan Schaeffer, regional managing director of the Interstate Commerce Commission in Philadelphia, said yesterday that his organization has spent about two weeks riding Colonial's buses and interviewing riders about their complaints, studying Colonial's records, and auditing Colonial's books.

"When we're all finished," Schaeffer said, "we'll make some recommendations." The ICC can seek court injunctions, hold quiet discussions, or go so far as to revoke Colonial's right to operate interstate trips.

Schaeffer went to a hearing on Colonial in Prince William County recently, he said, and "I have never seen more angry commuters in one place. We've heard some gruesome stories."

The federal Bureau of Motor Carrier Safety has been inspecting Colonial's buses for safety defects. Penn said that five buses were ordered out of service by federal inspectors, then returned when defects were corrected.