Under a truce reached yesterday with activists for the homeless, Metro officials will donate an old bus, to be heated and equipped with a toilet, for use as an overnight shelter for people who have been fenced out of the Farragut West station.

Separately, Metro officials disclosed that they have worked out an agreement to sell almost an acre to the City of Alexandria, which plans to build a 65-bed shelter for the homeless and an alcohol and drug detoxification center on the site.

The bus "really is a very rotten answer" but will do as "an emergency solution to an emergency problem," said Mitch Snyder, one of eight activists fasting to protest the gates installed by Metro to keep out the homeless who had been sleeping and relieving themselves on the station's escalators at night.

"What people really need is housing, but a bus is superior to a stairwell, which in turn is superior to nothing," said Snyder, who began fasting Nov. 9.

Snyder said he and the other hunger strikers, members of his Community for Creative Non-Violence, will halt their fast when the bus is available or the gates are removed.

Metro expects the American Motors General, a 40-foot bus at least 10 years old, to be ready Monday night to accommodate up to 25 persons, Metro board Chairman Joseph Alexander said at yesterday's board meeting.

Members of the local business community, led by Washington developer Oliver T. Carr Jr. and the Federal City Council, have agreed to pay $8,700 to refurbish the bus by removing seats and installing a toilet, Alexander said.

Carr was recently named chairman of D.C. Mayor Marion Barry's advisory group on homelessness.

Carr could not be reached for comment, but one source familiar with the negotiations said the financing arrangement consists of oral promises. "No one has pulled out a checkbook," he said, asking not to be identified.

The bus will be operated by the District government and Christ House, a nonprofit group that works with the homeless.

The bus will be parked close to the Farragut West Metro station from 10 p.m. to 6 a.m. during the winter months, and will be staffed by members of Christ House, said Metro board member Gladys W. Mack. The bus will not have beds and will admit men and women, she said.

The bus is intended primarily to serve the homeless who gather overnight at Farragut West. However, it could attract other homeless persons who settle in each night at other downtown Metro stations, seeking warmth rising from underground tunnels.

Alexander said that if the bus idea is successful and the District requests additional buses, Metro would consider donating more.

The bus to be used is among many that are out of service and considered surplus.

Mobile shelters are not a new idea. The District plans to buy five mobile trailers as shelters this year.

Disaster relief agencies often use trailers as emergency shelters, Snyder said.

"You probably will be seeing more use of mobile shelters," Snyder said.

"They don't require a building and can be put where the need is," he said.

The bus, however, is "not the model for the nation" as a solution for the problem of the homeless.

Because the old buses are considered to have no value, Metro can give them away. However, highly valuable Washington area property, purchased by Metro with federal funds, cannot be sold at less than its market value under federal law.

The process of making that determination has delayed by almost 10 months Alexandria's purchase of Metro's property on Mill Road near the Eisenhower Avenue station, according to sources familiar with the negotiations.

The 40,000-square-foot parcel is estimated to be worth more than $1 million, said City Manager Vola Lawson.

The city has applied for a federal grant of $754,679 to buy the portion that will be used for the shelter, which it hopes to open in January 1989, she said.

The process of seeking appraisals of the land's market value and settling on a sale price could take a year, but Metro has agreed to let the city start construction work next month, Lawson said.

The city had hoped to get the land for about 25 percent of its market value through another federal program and spent almost a year exploring that possibility.

City and Metro officials were told recently that the land must be sold at market value, with 80 percent of the proceeds to go to the federal government, which contributed that portion of the money used to buy the land.