Federal officials have granted Metro $35 million to buy land and construct two parking lots along the proposed Yellow/Green Line rail segment from Fort Totten to Greenbelt -- a move that Metro officials say will build momentum toward completion of the full 101-mile rail system.

However, the U.S. Department of Transportation also had some bad news for Metro yesterday: An application to buy 70 to 80 new buses was turned down for lack of funds.

Metro is heartened by the Greenbelt money because the Reagan administration has said since taking office that it could commit itself only to completing 75 miles of track, though it did not rule out finishing the rest at a future date. The segment to Greenbelt appears to lie beyond 75 miles.

But Metro saw its applications for the land and parking lot grants as a test, because they represented the first time the administration had been asked to fund a project beyond 75 miles.

Transportation Department spokesman Tom Blank said the decision does not change the 75-mile policy. The lots were funded because they could be used for satellite parking without the rail line, he said, and money for the land was approved to preempt other development that would make eventual construction more costly.

Still, Metro sees the decision as a victory for its goal of beginning work on unstarted segments of the system as quickly as possible. Partially finished track is less likely to be canceled than track that exists only on planners' maps, Metro believes.

The grants are included in $284 million Metro got for rail construction in fiscal 1982, which ended last week. That was $31 million less than it had requested. But given administration efforts to trim federal spending, Metro officials feel the system did well.

"Every year like that puts us closer to a 101-mile system," General Manager Richard S. Page said.

The grants include $15.6 million for land for right-of-way and stations in Prince George's County along the full length of the proposed line between Fort Totten and Greenbelt. Metro's notoriously unreliable construction schedules have the segment opening in the early 1990s.

Metro also got $2.2 million for a 500-space parking lot at West Hyattsville and $17.2 million for a lot of similar size at Greenbelt and special access ramps to it. Pending completion of the rail segment, Metro's plans have the lots providing satellite parking with commuters going on by bus to operating rail lines for the trip downtown.

Metro has resisted defining a 75-mile system. But current timetables would extend the 39 miles now in operation to about 75 by the late 1980s.

New Blue/Yellow Line track from National Airport south to Huntington would open and Yellow Line track between Gallery Place and the Pentagon would open in late 1983 or early '84. The Red Line would open through to Shady Grove a year or so later and the Orange Line to Vienna in 1986 or later. In the late 1980s, the Green Line between U Street and Anacostia and the Red Line to Wheaton would open.

That would make about 75 miles. After that the following would remain: The Yellow/Green Line between U Street and Greenbelt, the Green Line between Anacostia and an as yet undecided site in southern Prince George's County, the Yellow Line between King Street and Franconia-Springfield, and the Red Line between Wheaton and Glenmont.

On the grants for new buses, meanwhile, Metro was told that its application for $10 million would be reviewed for the fiscal 1983 program. The refusal is not a big setback for Metro's efforts to improve bus service, because its most important component is a plan to strip and rebuild 600 old buses. Money for the plan's first year has already been approved.