Metro will take another giant step toward rebuilding the area's transportation system Tuesday morning when it changes or eliminates about 150 bus routes and pushes 15,000 commutes from them onto the subway extension to Silver Spring.

If past experience with similar changes is any guide, Tuesday will be mass confusion. Buses that used to go here will go there or stop someplace in between. Route numbers will change.

Commuters who have never had to deal with the subway before will be introduced to the wonders of Farecard, the automatic fare-collecting system. They will learn that they cannot transfer from bus to train without paying two full fares, but that they can transfer from train to Metrobus for free, unless their tip ends with a bus ride in Virginia.

Added to this is the massive expansion by Mountgomery County of its neighborhood small-bus system called Ride-on. Ride-On will replace many Metrobus routes in Silver Spring, Takoma Park and other down-county areas and provide a demonstration project for other localities that might be interested in running their own bus companies.

In short, there will be a major revolution in the public transportation patterns east of Rock Creek Park in both the District of Columbia and Maryland and north of Rhode Island Avenue in the District.

The cutbacks in Metrobus service are being made in the name of economy and common sense. The primary purpose is to eliminate as much parallel bus and subway service as possible and to change the buses from a long-haul mission into feeders for the subway.

There are losers in this revolution. Some people who had good bus service will find that public transpotation has become a costlier pain. Once convenient one-bus trips will involve a transfer to another bus or to the train.

But for the vast majority, Metro believes, the tie-in of the bus service with the two'week-old Red Line Metro extension from Rhode Island Avenue to Silver Spring will provide swifter, more comfortable service.

The bus route changes technically took place just after last midnight. Since Monday is a holiday, Tuesday will be the first regular workday for the new bus schedules. The subway will not run Monday.

Metro has gained an average of about 17,000 riders a day on the subway system since the Red Line to Silver Spring opened Feb. 6. That brings average daily ridership to over 160,000 a day. Metro had predicted that the opening of the Silver Spring extension and the bus changeover would bring 30,000 new riders to the subway system by March 6 and 40,000 by June 30.

The cost of a transit trip will vary from person to person under Metro's Rube Goldberg fare structure. Generally, round-trip fares for Maryland residents who use both the bus and subway for one trip will be about the same or no more than 10 cents higher than the all-bus fare for the same round trip.

FOr District of Columbia residents who take the bus and subway, however, the cost of a round trip will be at least 30 cents higher. In almost every case it will be possible, but less convenient, to make the trip entirely by bus.

Metro or Montgomery County employees will be at all of the four new Metro stations Tuesday to explain transfer procedures and fares and provide other information.

Commuters from Virginia, Southeast Washington and southern and eastern Prince George's County went through the same thing in August and September. Then, bus routes intersecting Metro's Blue Line were cut back at outlying Metro stations.

Early returns on that cutback show that Metro retained most of its transit riders at the two main Virginia transfer points - Rosslyn and the Pentagon - but that there was a decrease from Southeast Washington and Prince George's County.

Part of the reason, according both to observation and complaints to the Post from users, is that the bus terminals serving those commuters at Stadium-Armory, Potomac Avenue, Federal Center SW and L'Enfant Plaza, are less than convenient. Additionally, the subway has meant double fares for the D.C. residents.

There has been some fine tuning of those changes since they were more and two or three of the more egreglous problems have been cured.

The worst of these, however, remains. That is the situation at the Potomac Avenue Metro station in southeast Washington where hundreds of Prince George's County commuters transfer from bus to subway and vice versa in the evening.

Because of geography, politics and money, commuters in the evening must tramp across six lanes of Pennsylvania Avenue traffic then stand in a field to catch their buses. Everybody thinks it's terrible and Prince George's County and the District of Columbia have worked out a theoretical solution, but it hasn't happened yet. Bad weather has prevented some construction that is needed to make the solution work according to D.C. Transportation Director Douglas Schneider.

All four of the new Metro stations on the Red Lin have convenient-looking bus-rail transfer facilites. The Silver Spring Metro station, which crosses Colesville Road and East-West Highway, will become the main transportation hub on the extended Red Line. Twenty-three Metrobus routes and 20 Ride-On routes will connect there.

Seven Metrobus routes and six Ride-On routes will connect at the Takoma station, 13 Metrobus routes at Fort Totten and 11 at Brookland.

Because of this newest cutback, Metro expects to reduce its bus fleet by 77 and to save $2.9 million annually in bus operating costs.

Montgomery County, by replacing Metrobus service with Ride-On, estimates it is saving about $1.5 million annually in subsidies to Metro.

However, the county is purchasing 40 new buses and is increasing its four-route Ride-On network to 20 routes. The new Ride-On service will cost $3 million to operate, the county estimates.

The Ride-On fare will be 25 cents. There will be no transfer between Ride-On or the subway at any time. There are free transfers from Ride-On to Rde-On and from Metrobus to Ride-On. A transfer from Ride-On is good for 25 cents off a Metrobus fare.

The county's Ride-On experiment has been successful in terms of attracting runs, and couty transportation director Richard Lynch is predicting 60,000 riders a week on the new Ride-On.

Other are jurisdictions have been studying Ride-on because of its smaller labor costs. Drivers account for almost 80 per cent of the Metrobus operating budget. Under the current Metrobus labor contract, part-time drivers cannot be hired. Ride-On uses part-time drivers.

Montgomery County has avoided some federal labor protection clauses by purchasing its buses without federal assistance. New bus purchases are normally eligible for 80 per cent federal funding.

Because of the vast numbers of bus route chances, Metor's information services have been swamped with calls. There have also been instances, reported to and confirmed by The Post, of the information service providing inaccurate data on where and when the new bus routes run.

MIchael Noonchester, of Metro's marketing office, said that phone calls had increased from about 45,000 a day to 48,000 a day in the past week. That does not count busy signals.

Once a caller gets through, he or she is placed on hold until the call can be taken in turn. About one call in five is never answered because the caller gives up, according to Metro statistics.

Noonchester conceeded that there were some instances of misinformation. "Part of the problem," he said, "is that our people are dealing with two sets of information - the bus schedules today and the bus schedules after the changeover. Normally our operators have to know about a 750 route system. Right now they're up to about 1,100."

New bus schedules and route guides are supposed to be available at the Metro stations and from the bus drivers. If the Blue Line experience last August and September is an example, it will take about a week for people to figure out how the new system works.