Mass transit's changing shape
* By early next year Miami residents will be shuttled around the traffic-choked downtown area in a pilotless ''people mover.''
* Seattle is studying a plan to use ''dual powered'' buses that will run on electricity when darting through a tunnel under the city and switch to diesel power in aboveground, outlying areas.
* Buffalo, N.Y., hopes to have the turnstiles spinning on the first leg of a 6.4-mile ''light rail'' system early next year.
American cities - their freeways jammed and civic pride showing - are sketching plans for a new generation of mass-transit systems.
But tight federal purse strings and concern about luring enough riders may stall many of the systems before they move beyond the drafting tables.
As a result, cities are being forced to consider innovative alternatives to costly rail systems and hunt for ways to finance existing projects.
No fewer than 33 American cities are now planning subway and other rail projects. Even if only a fraction are completed, it would represent a significant addition to the nation's urban transportation network: Fewer than a dozen have been built in the past 40 years.
''There is an amazing boom going on,'' says Dr. Vukan Vuchic, a transportation expert at the University of Pennsylvania.
Plans for many of the rail networks were laid back in the 1970s, when federal aid for new systems was abundant. But others have been sketched more recently, particularly in the West and Midwest, as freeways have become more congested.
At the same time, there is an element of prestige and keeping-up-with-the-Joneses involved in the rush to lay rails: Many cities view mass-transit systems as central to spurring economic development and don't want to be left behind as their urban brethren lay tracks.
By far the most popular route is to build the light-rail systems - updated versions of the old trolley-car lines. Heavy rails usually run partly underground and require a separate right of way. Light-rail setups can cross or run on streets.
Light rail is also cheaper and more flexible - but far from inexpensive (they can cost $25 million a mile to build; heavy rail, as much as $100 million). Subways, to be sure, haven't completely disappeared from planners' portfolios: Dallas and Los Angeles are planning major new systems.
Extensions remain to be laid on new systems in Washington, Baltimore, and Atlanta, among others. But most cities, from Pittsburgh and Buffalo to San Jose, Calif., and Portland, Ore., are embracing light-rail technologies.
A few cities, too, are pushing ''people movers'' - automated trains that shuttle around on tracks similar to those at some airports - to ease congestion in downtown areas. Next year Miami expects to be whisking some 40,000 people a day around on two 1.9-mile downtown loops. Jacksonville, Fla., and Detroit are eyeing similar systems.
Seattle is taking a different approach altogether. It wants to try special buses. The idea: build a 1.25-mile, $250 million tunnel leading out from the central business district. Then run buses through it - operating electrically underground and on diesel power at street level. Eventually, if the city can afford it, light-rail systems would be laid along the same routes.
Scarcity of federal funds, though, may crimp many of these projects. In the past the federal government has put up 75 percent or more of the cost of new systems. States, too, have usually made contributions, often leaving cities carrying little of the financial burden. Cities now have plans for some $18 billion worth of new rail systems. Yet only about $400 million in federal funds will be available this year for new starts. Los Angeles alone wants $336 million for its proposed 18.6-mile, $3.3 billion subway system.
The Reagan administration, through the Urban Mass Transportation Administration, is tightening the guidelines governing who will receive federal aid. The agency wants to help underwrite only those cities that can justify the need for new systems - and drum up plenty of local funds for building and operating them.
Behind this move lies the belief among many transportation experts that there are other ways to move people about than on expensive, inflexible rail systems. They argue that young cities in particular should consider such alternatives as buses and cheaper trolley systems, since land-use patterns in these locations usually aren't fully developed.
At the same time, many point out that shifting demographics undermines the rationale for laying lines in many areas: With so many people moving to the suburbs and beyond, there sometimes isn't the ''critical mass'' of people to support new systems. ''Most central cities have lost population,'' says Gerald Miller, acting director of the Urban Institute's transportation program in Washington.
A few cities are already finding the difficulty of rail-building today: The projects in Buffalo and Miami have run into construction problems, and Detroit's downtown people mover, begun last year, is coming in well above its original $ 145 million price tag.
Some communities are trying to keep projects on line without reaching so deeply into Uncle Sam's pocket. Many - Los Angeles and Atlanta, for instance - are turning more to sales-tax revenues to fund transit improvements. Some want private developers that benefit from locating near rail lines to help fund projects. Dallas believes it can build a subway system with virtually no federal aid.
Creative financing, though, won't ensure that all projects are built. Experts say that many cities will have to develop cheaper, better-tailored travel patterns. This would include improved coordination of bus, rail, and other transit services, as well as more van- and car-pooling, ride-sharing, and express-bus lanes.
Other ideas gaining in popularity: ''dial-a-ride'' programs, where commuters call up for cabs or vans, and ''subscription'' bus services, where residents contract for rides by the week or month. What seems certain, says C. Kenneth Orski, president of Urban Mobility Corporation, a Washington, D.C., consulting firm, is that the days of the big, monolithic transit systems like New York's and Chicago's are gone. He sees less stress on new technology and more on ''innovation in management and service.''