Montgomery County officials got a preview yesterday of the rail service between Friendship Heights and Grosvenor stations--a trip the public won't get to take until late 1984.

County executive Charles W. Gilchrist and members of the council flipped the switch to "electrify" the long-awaited Red Line extension and then, with Metro staffers, curious reporters, and a few photographers sped at up to 75 miles per hour to the station at Friendship Heights.

In taking the trip, planned to show off the line, the officials seemed to realize they could be stirring the seeds of popular unrest.

"Could you imagine someone driving by and seeing this train running, knowing it won't be open to the public for two more years," asked Council member Scott Fosler. "It's really hard to understand why we have to wait two years," said Council President David L. Scull.

"It's unfortunate we can't open it sooner, but that's the way it is," said Tad Weigle, Metro's assistant general manager of operations. The biggest problem, he said, was that they don't have the 94 additional cars needed to begin service.

The cars were held up by a workers' strike at the Pittsburgh plant making the brakes for the cars, he said. That strike delayed ordering the cars from the Italian manufacturers.

Even if the cars were available, Metro officials pointed up other problems in opening the line. Commuters would descend on Grosvenor with no place to park, and Metro would have no place to store the new trains, they said.