Rome, the textbooks tell us, wasn't built in a day, and the tortoise-like progress made in recent years by the builders of its new subway may be proof that history does, after all, repeat itself.

Still-buried remains of the ancient imperial construction that put this city on the tourist's map bear part of the responsibility for the slow going that in 12 years has produced less than nine miles of tunnel and track.

Skyrocketing costs, financial confusion and bureaucratic complexity have done the rest. Experts have calculated that at the present rate, the original four-line, 65-mile subway project placed on city drawing boards back in 1962 would take another 50 years to complete.

A and B lines are to meet at Rome's central train station to intersect the city in a huge X. Service on these lines is now scheduled to begin in late 1978, although some skeptics insist 1979 or 1980 would be more realistic.

On 6-mile tretch begun by 1/2 Musolini has been in operation since 1955. So completion of A and B will give the city a 21-mile system with 44 stations. City engineers say this will take some of the pressure off the crowded downtown area, whose traffic jams were immortalized by Federico Fellini in his film "8 1/2."

But C and D, two east-west lines, will probably never run.

First estimates were that the entire system could be built for $60 million. The nine completed miles of A line have cost more than that. City planners are now seeking other alternatives to alleviate traffic chaos in this city of 3 million.

"My daughter will not have the pleasure of seeing her father complete this project," said a white bearded senior city land surveyor. He claimed it was the Metropolitana that turned his beard white.

Officials of the leftist governments that run both the city and the surrounding Lazio region tend to agree with the criticisms.

"At today's cost of $35 million per kilometer [0.6 mile] we simply can't afford to continue," said Alberto Di Segni, the Lazio region's Socialist transport commissioner."We've got limited resources and prefer to build day-care centers and supplement our double-and triple-session schools."

Vittorio Masci, a city engineer who has been working on the subway for more than 20 years, said that, after the completion of the two lines now in construction, "No plans for further building have been made."

City planners feel the reduced system will be sufficient to link outlying populous areas to one another while speeding up transit to the center. The orange trains of line A and the light blue convoys of line B will run about once every 1 1/2 minutes, giving each line an hourly capacity of as much as 48,000 persons and enabling passengers to go from one end of Rome to another in less than half an hour.

The main goal is to ease pressure on the city's 2,500 buses and to persuade Romans to stop driving their cars into town. Almost 1.5 million continue to do so daily, despite rock-bottom bus fares of about five cents and soaring gas prices, now at $2.40 a gallon.

Past projects to free Rome's congested downtown area have had only partial success. Wily motorists enter "bus-only" lanes when no one is looking, and find ways to avoid policemen stationed to prevent access to parts of the historic center where private vehicles are banned.

Huge parking lots with low rates are to be built at each of the subway's four terminals. Other plans call for the construction of terminals for intercity buses on the outskirts. Passengers will save about 45 minutes traveling time by transferring to the Metropolitana.

Additional complements to the system include the construction of permanent barriers to block private cars from streets reserved for city buses.

City officials admit that the subway construction is pace of one kilometer a year lags behind world standards. They explain that underground work in a place like Rome is tricky at best. The densly populated city has a jungle of underground networks - gas and waterpipes, telephone lines and four electricity systems, not to mention a couple of old aqueducts - which must first be moved and then reconnected.

Another problem has been the continual discover of new archeological treasures that have required rerouting. One Piazza chosen as a station site was discovered to conceal an ancient bath. "If we'd had the money we could have worked the station into it, but with our limited funds such luxuries are out of the question," an official said.

Protests by merchants in neighborhoods where open-air construction disrupted business created other difficulties. "In one area we lost three years and then had to switch to more expensive underground construction," said Masci.

Every change in the original plans - rerouting, refunding or rediscussion of the relative merits of the third rail and overhead electric lines - caused months and even years of delay, according to Masci.

One disgruntled Roman, who after years of empty promises is convinced the Metropolitana "is only a road to nowhere," says that so far the traffic department's subway plans have only made things worse.

"The idea is to improve traffic circulation," he complained, "but with so many streets closed off or piazzas narrowed, most of the city's major bottle-necks in recent years have been caused by the Metropolitana itself."