* Heavy cargo will float between major Brazilian cities in 150-ton dirigibles.
* Passenger trains will race along a single elevated rail between here and Sao Paulo, 250 miles to the southwest, at speeds of up to 150 miles an hour.
* Commuters will zip silently along urban thoroughfares in bullet-shape coaches moved by a high-speed jet stream.
So say transportation researchers in Brazil.
While alternative-energy projects, such as the wide use of alcohol in automobiles instead of gasoline, have generated considerable enthusiasm in oil-poor Brazil, alternative transportation proposals continue largely unnoticed. Yet new transit projects may be just as important for the energy economy of Brazil's future as alternative means of supply, experts agree.
Two of the most interesting alternatives now being studied here are the dirigible and the pneumatic train.
Lighter-than-air craft got a bad name when the 247-foot-long German zeppelin Hindenburg crashed and burned at Lakehurst, N.J., in 1937.
The dirigible of the '80s, however, will be almost risk-free, proponents say. For one thing, highly flammable hydrogen gas will be replaced by nonflammable helium to keep the craft in the air.
A group of Brazilian engineers working under the auspices of Embraer, the country's government-owned aircraft builder, has developed a plan that could put the first Brazilian-built dirigibles in the air by 1985.
The engineers would like to build two 20-ton, helium-floated craft by then for use in experiments.
''There is no other alternative to the dirigible in about four-fifths of our national territory,'' says project chief Carlos Barroso, ''because there are too many places where it is simply not economical to extend highways.'' Mr. Barroso says the lighter-than-air craft would use ''almost no combustible fuel'' and, although the huge craft travel slowly (45 miles an hour), they can carry more cargo than existing aircraft.
Dirigible proponents say the craft are quiet and nonpolluting; can take off and land vertically; and can go just about anywhere.
The Brazilian Air Force, Embraer's parent agency, is skeptical. Arguing that the Air Ministry has to cut costs because of Brazil's high inflation rate, the dirigible project did not receive its expected appropriation last year and development of the craft may be delayed until the late '80s or beyond.
The pneumatic train, on the other hand, seems to have sparked the imagination of government money men.
The Transportation Ministry spent $500,000 in 1980 on an experimental model, and the southern city of Porto Alegre, about 600 miles south of Rio de Janeiro, is expected to build a 4-kilometer-long (2 1/2-milel) pneumatic train line downtown.
The father of the train is Oscar Coester, a former airline executive, who founded his own engineering firm five years ago to develop the idea.
''The idea came to me in the middle of the night,'' he says.
The train, according to Mr. Coester, is as simple as vacuuming a rug. In fact , that's where he says he got his idea. The train car is streamlined to the point where it looks like an elongated bullet and runs on a single track.
The train's operating principle is based on suction. Each of the coaches is hermetically connected to an air conduit in the middle of the track. The flow of air in the conduit, produced by huge fans, sucks the coach along the track just as a vacuum cleaner sucks up the dirt from a carpet.
The train has no driver. A single person sitting in a control room regulates the flow of air for all the trains on a given line.
Coester says that the trains can travel at speeds of up to 150 miles an hour and that they produce no pollution and almost no noise. The only sound is a faint pneumatic whoosh.
The technology, Coester asserts, is ''100 percent Brazilian,'' and it will cost about $2.3 million a kilometer to build the specially designed tracks, compared with some $45 million a kilometer for the new subway in Rio de Janeiro.
Brazil's relatively strong electric power position, based on its extensive hydroelectric network, and a lack of proven oil reserves have made electric-powered mass transit a significant option again.
Two Sao Paulo engineers, Vicente Machado and his son Benito, have developed an experimental electric-powered monorail which, they say, will be cheaper to build and operate than anything now available in Brazil.
The chief advantage of a monorail, they contend, is its narrowness.
Any public thoroughfare wide enough to have a median strip is fair game for monorail builders since the base of the monorail's supporting pillars is only 20 inches wide. The Machados say it would be possible to build a monorail system with a top speed of up to 150 miles an hour in almost all of Brazil's big cities without the expropriation of any private land.
Further, they add, the monorail coaches are so streamlined and so light that they would use less energy per passenger-mile than any other form of mass transit now in use in the country.
The monorail builders have not yet found a buyer, however. Looking around, they say that Sao Paulo; Curitiba, 300 miles to the south; and Porto Alegre are good possibilities.
''This is the sort of thing we should be doing,'' says the Curitiba mayor, Jayme Lerner.
''In our city, individual transportation is responsible for 74 percent of our consumption of gasoline,'' he concludes. ''Public transport uses only 8 percent.''