In the history of rapid transit the United States, almost nothing was built between 1928 and the 1970s. (Cleveland's heavy rail the noted exception*). And for good reason--demand for transit collapsed. But in 1970, we recognized we had 'an urban transportation problem', which is the preferred euphemism for the explosive growth in traffic congestion.
Transit was the clear solution. But even the solution was a problem. Private companies had built most of the transit infrastructure before 1928, and municipalities getting into the game only in response to their failure. But no private entity was willing to build transit in 1970--competition from the automobile was just too fierce. Conservative pundits love to argue that this reflects the innate attractiveness of the automobile. They also love to ignore the billions of dollars in Federal subsidy provided for the Interstate Highway system. So, it order to compete with the automobile, transit projects required a subsidy. Given that the 70's were sort of the high-water mark of 'big government' and centralized planning, it became a Federal project.
None of them did terribly well, at least at first. They had very high costs per rider.** They've done better over time, as traffic congestion has worsened, and developers have responded to the accessibility premium of locations near them, so more things are near them. BART won its spurs when the Bay Bridge collapsed. The DC metro has become the most-ridden transit system in the US.
Many of the characteristics of these systems would carry over into later light rail planning and systems.
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*Planned 1920, partially built 1929-1930, finished 1955-1958.
**For some of the systems, for some years, it would have been better to either a) buy everyone a cheap car, or b) put all of the money into buses.
***DART, despite the similarity of names, is not actually part of the M/ART transit cycle, but part of the 1982+ light rail cycle, starting operation only in 1996. (Initial efforts to fund it began in 1983).