The beginning of regular Metro subway service in Silver Spring tomorrow is but a prelude to a major change in public transportation commuting habits for the northern half of the Washington metropolitan area.

The addition of rapid rail service will be followed two weeks later by a restructuring of the Metrobus system, just as was done in the southern half of the region after the opening of the Blue Line last summer.

About 150 bus routes - half in Maryland and half in the District of Columbia - will be eliminated or revised effective Sunday, Feb. 19. The first day of real impact on most riders, however, will be Tuesday, Feb. 21. The intervening Monday is a federal holiday.

The purpose of most of the changes is to eliminate bus service that paralels the subway line That practice is being followed to save money because both the bus system and the subway operate with a taxpayer-reimbursed deficit.

The reductions in the miles buses travel have resulted in a smaller Metrobus fleet. Most than 200 buses are being cut from a fleet of 2,000 over a three-year period.

Silver Spring will become the biggest bus terminal on the new Metro line, but Takoma, Fort Totten and Brookland will have bus terminals and many routes will stop at those station as well.

Additionally, Montgomery County is expanding its neighborhood small-bus network, called Ride-On, and is replacing some Metrobus service in the Silver Spring area.

The most initially confusing aspect for the commuter forced to transfer from bus to subway is the transfer procedure itself.

Here are the rules for riders whose public transit trips begin or end in Maryland or the District:

It is impossible to transfer from bus to subway. The rider boarding in Maryland and planning to transfer at Silver Spring should pay the bus fare only as far as Sliver Spring, then buy a farecard for the subway ride.

It is easy to transfer from the subway to a Metrobus with a destination in either Maryland or the District of Columbia. The rider enters the subway station with a farecard, then obtains a free bus transfer from a vending machine before entering a train. That transfer will permit the rider to board a bus free at the Metro station where the rider leaves the subway, no matter how far the bus is going.

It is important to remember to get the bus transfer at the Metro station where you enter, because bus drivers are instructed not to accept transfers stamped with name of the station closest to the bus stop, hat procedure was adopted by the Metro board to reduce transfer abuse.

For most Maryland residents the resulting round trip cost of the trip using bus and subway will be about the same as the all-bus trip costs now. For most D.C. residents, however, the resulting round-trip cost will be about 40 centers higher.

The realignment of the bus system is made somewhat more confusing by the fact that although many routes are being eliminated or renamed, most of the route designations will survive.

For example, although there is no longer a 4 bus in the District of Columbia, that route designation will survive in Maryland for a new route. But the new route will be the old Maryland Route T7. And the D.C. route will be called E2, another old number.

Metro's phone information service can provide specific, route by route data. The number is 637-2437. It is frequently busy.

More specific information will be handed out on the buses beginning Feb. 1.

Here is a route-by-route listing of the changes. If the bus route is not listed, no change is scheduled.