The Bay Area Rapid Transit system transports nearly 400,000 passengers on an average weekday. It's the nation’s fifth-largest rail system, with 104-miles of track stretching from the far reaches of the Bay Area's eastern suburbs to San Francisco International Airport, south of the city.

So when trains don't run, things get real messy real quick.

Which makes it all the more surprising that BART hasn't been around for that long: operations started just over 40 years ago. Compare that to New York City’s much larger subway system, that started running in 1904, or Boston’s even older rail network that first rolled in 1897.

So how'd we get around before BART?

The Key System

In 1903, a privately-run mass transit network called the Key System (or Key Route) began providing bus and streetcar service in Oakland, Berkeley and various of other East Bay cities. In the 1940s and 1950s, the network also operated regular commuter rail service to San Francisco via the lower deck of the Bay Bridge. But by 1958, in the face of booming highway construction and rapidly rising car ownership rates, transbay service was dismantled. (The Key System: San Francisco and the Eastshore Empire, by Walter Rice and Emiliano Echeverria, provides an interesting history of the system.)

Two years later, a newly formed public agency called AC Transit bought the Key System's existing East Bay bus routes (the streetcars had been phased out by 1948).

Check out this lengthy but heartily entertaining Key System promo video ... with a nice little revisionist history of California, replete with descriptions like this: "Life in Spanish California was leisurely and gay. A warmhearted people in a kind and bountiful land expressed themselves in colorful fiestas. Where … lovely senoritas and dashing caballeros danced the exciting steps of old Castile."

BART is born

By 1950, nearly 2.7 million people lived in the Bay Area, about a million more than the previous decade. It was the beginning of a regional population boom that would continue to grow by roughly a million people every decade for the next fifty years, guaranteeing increasingly heavy congestion on roadways.

A state commission created years earlier to study the Bay Area's long-term transportation needs, recommended the construction of a five-county rapid rail network linking major commercial centers to suburban communities.

In its 1957 report, the commission said:

A segment of the transbay tube is lowered into the Bay (courtesy of bart.gov). — A segment of the transbay tube is lowered into the Bay/BART.gov

"If the Bay Area is to be preserved as a fine place to live and work, a regional rapid transit system is essential to prevent total dependence on automobiles and freeways."

The original plan included San Mateo and Marin counties, both of which eventually bailed amidst cost concerns (as well as controversy over the feasibility of running a line across the Golden Gate Bridge). The final proposal included a 71.5 mile electric rail system with 33 stations in 17 cities spread across the counties of Alameda, Contra Costa and San Francisco. Among the largest public works projects in history, the total cost was projected at just shy of $1 billion, with the brunt of funding from a bond measure approved by voters in the 1962 general election. The project ended up with a price tag of more than $1.6 billion.

BART construction officially commenced in June 1964. President Lyndon Johnson presided over the groundbreaking ceremony of the 4.4-mile Diablo Test Track between Concord and Walnut Creek. By January 1966, construction of the Oakland subway began, and in November of that year, the first of 57 giant steel and concrete sections of the 3.8 mile transbay tube was lowered to the bottom of the Bay. The tube was completed in August 1969, and for a brief window, the public was allowed to bike and walk through it.

Big wheels start turning

After 466 days, workers completed a 3.2-mile bore through the hard rock of the Berkeley Hills in February 1967, and that summer, crews started construction of a subway section about 100 feet below Market Street in San Francisco.

BART began offering service to the public on September 11, 1972, on a 28-mile segment between Fremont and MacArthur stations. The day before, the Oakland Tribune published a 40-page special section, declaring: "BART is no longer a dream. It's here and it's yours."

Transbay rides, however, didn't begin until September 16, 1974.

In the end, the monumental project took many years longer and hundreds of millions of dollars more to complete than originally anticipated.

Today the system has 44 stations serviced by a 669-car electric fleet that many considered downright revolutionary. In 1973, the first full year of operation, BART's average weekday ridership was about 32,000. By 2014, the average had risen to 403,680, totaling roughly 122 million trips annually. The system had its highest daily ridership on Nov. 3, 2010, the day of the Giant's World Series victory parade, with 522,200 recorded exits. This summer's Warriors victory parade was a close second, with more than 551,000 rowdy riders.

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You Can't Ride BART Across the Bay This Weekend So May As Well Learn Something About It Instead