The Richmond, Fredericksburg and Potomac Railroad last night showed the Alexandria City council plans for a $45 million motel and business complex that the railroad said would revitalize the area around the future Metro station on upper King Street.

But the council immediately threw the project into doubt by voting to place a 77-foot height limitation on construction in the area.

The railroad's plans call for construction of several 100-foot-tall buildings including a 300-room motel, a conference center seating up to 800 persons, two or three office buildings and a 1,000-car garage.

The development on land the railroad owns near the 343-foot-tall George Washington Masonic National Memorial, would be bounded by King and Duke streets and Diagonal and Callahan roads.

Developer Donley F Hunt, who has formed a partnership with the railroad to build the complex on the 10-acre site, said he did not know whether the railroad would contest the new height limitation.

After last night's meeting, council member Donald C. Casey called the plans "not in keeping with the city's master plan for the area." Casey said the plans called for "far more density of people and cars than anything the city has proposed."

Vice Mayor Robert L. Calhoun questioned whether the railroad could obtain the construction for some of its porposed buildings, which would be built over the tracks of the Metrorail system. The so-called air rights are owned by the Washington Metropolitan Area Transit Authority. Hunt told the council the complex would "serve as a catalyst for the entire upper King Street area," which contains a variety of small stores, auto garages and emplty lots. Hunt said later that he had not obtained air rights from WMATA but that he is negotiating with them.

The site is adjacent to the Rosemont neighborhood of predominantly single-family homes. The site is occupied by the Rf&p Railroad Station, which is used by Amtrak trains. The station, built in 1906, would be maintained in its current state at the center of the complex, Hunt said.

The towering Washington Masonic memorial, one of the city's most conspicuous landmarks, stands across the street from the proposed complex. It is built on a hill, but some of its lower sections would be blocked from view by 100-foot buildings.

Enactment of the 77-foot height limitation came after bigorous campaigning by residents of the area to lower the maximum building height from 150 feet.

The plans were drawn up by architect William Vosbeck, who told the council that the parking garage would be placed on the south side of Duke Street, keeping traffic away from the congested King Street and the residential Rosemont neighborhood. The council scheduled no further consideration of the rpoposals.

In a related development, the council also upheld the denial by the city planning commission of a building permit wanted by a developer for two 100-foot buildings in the same area. Harry P. Hart, attorney for the Pisner Eig development firm, said he did not know if his clients would sue the city for their refusal to grant the permits, which are needed before construction can begin.

'Mayor Charles E. Beatley Jr. and Councilman Carlyle c. Ring Jr. were absent from last night's meeting.