The story
The San Francisco Bay Area Rapid Transit District (occasionally abbreviated in early years to BARTD) was created in 1957 to provide a transit alternative between suburbs in the East Bay and job centers in San Francisco's Financial District as well as (to a lesser extent) those in Downtown Oakland and Downtown Berkeley.
Of the six Bay Area counties initially envisioned as participants—San Francisco, Marin, Contra Costa, Alameda, San Mateo, and Santa Clara—only Santa Clara County refused to join when the district was first set up. From 1957 to 1962, the district originally included the other five counties, including both San Mateo and Marin. San Mateo opted out in 1962, preferring to utilize funds to build its freeway and expressway system. Marin left a month after San Mateo, fearing that it would be unable to absorb its share of operating costs with San Mateo's withdrawal. Marin was also concerned by ongoing debate about the feasibility of running trains across a lower deck of the Golden Gate Bridge.
From Wikipedia’s article on Bay Area Rapid Transit District, shared under CC BY-SA 4.0.5