Alexandria is finally talking nuts and bolts about neighborhood buses.

Years after a city-owned transit system was first proposed, officials are evaluating proposals from three companies vying to supply 17 buses that could begin carrying passengers at yet-to-be-decided fares on four major routes early next year. They hope to take a recommendation to City Council next month.

The goal is to inaugurate service at the same time or shortly after the opening of the city's three Metrorail stations, which is scheduled for December but expected by many to be postponed. "We're going to make every effort to start it whenever the Metrorail starts, even if we have to lease buses," says Mayor Charles Beatley, who has promoted the project.

The idea is to have 30-foot, air conditioned vehicles feed commuters to Metrorail, providing a customized, responsive type of public transportation through commercial and residential corners of the city that the larger Metrobus system passes by.

Equally important, the buses are meant to save money the city is now paying to Metro in subsidies. Alexandria has budgeted about $6.6 million for Metro operating subsidies this year--a city-operated bus system could reduce that by about $350,000, officials say.

In short, Alexandria is gambling it can do the job better and cheaper on its own. It is a risky venture, requiring the purchase of costly vehicles, maintenance equipment and the hiring of a new work force of drivers and mechanics, but the city is willing to take the chance.

Hopes for saving money hinge on avoiding two things blamed for driving up the cost of Metro: unionized drivers and mechanics paid union wage scales, and federal aid, which brings with it such costly requirements as heavy paperwork and wheelchair lifts.

Alexandria's project is part of a trend among local governments in the Washington area to secede in part from Metro, one of the area's few regional ventures.

Montgomery County already operates its "Ride-On" bus system, while Fairfax County is working toward a feeder bus system of its own around the Huntington Metrorail station. D.C. has experimented with contracting directly from Metro for locally tailored "Metro-Mini" service.

No final decisions on routes have been made for the Alexandria system. But as now conceived, four city routes--designated A-2, A-3, A-4 and A-5--would ply streets from Old Town to the West End.

At rush hour, commuters on Washington Street would find a city bus or a Metrobus moving along King Street every six minutes and every eight-and-a-half minutes during off-peak hours. "Our goal is to maintain existing headways (the time period between buses) during peak period and we would like to improve them slightly where possible," says city official Carol Kachadoorian.

The routes would bring about the following changes in Metrobus service:

* The 25A between Ballston and Old Town would end at Northern Virginia Community College.

* The 6B between Landmark shopping center and the Pentagon would be eliminated. The 6A, now operating between Landmark and the Pentagon, would only go as far as Bradlee shopping center in the Shirlington area.

* The 12, 14 and 15 buses between Old Town and the Pentagon would be eliminated.

* The 10S and 10T Metrobus routes would be shifted slightly.

* The 28A, which operates between Hunting Towers and Tysons Corner, would be eliminated. The 28B would be rerouted to terminate at Wythe and Pendleton Streets, rather than Hunting Towers. A proposed new Metrobus route, 28C, would operate during rush-hour only between Southern Towers on Seminary Road and Old Town.

The job now is to buy buses. The City Council has approved $2.1 million of city funds to be spent for buses, parts and equipment. Earlier this year, the city advertised for contractors. Three responded and their proposals are being evaluated.

City transportation planners are leaning toward diesel-powered, 30-foot buses, with seats for up to 33 people. Ten feet shorter than standard Metrobuses, the vehicles would be less obtrusive to the residential environment and easier to maneuver.

City transportation planners want to use Metrobus' basic fare, 75 cents. But some thought is going into the possibility of setting up transfers to discount the fare for those using Metrorail or to provide cheaper short-distance rides within Old Town.

Beatley has criticized the staff's choice of buses. He prefers something close to those found in West European cities--buses with floors close to the ground, to facilitate getting on and off. The decision will be up to City Council.