The authors intend to study access to different wage levels of jobs and also to compare to car access. Their study included bus as well as rail, but some of their maps show footprints of urban-rail lines.This study examined the accessibility to jobs by transit in 46 of the 50 largest (by population) metropolitan areas in the United States. It is the most detailed evaluation to date of access to jobs by transit, and it allows for a direct comparison of the transit accessibility performance of America’s largest metropolitan areas.
Rankings were determined by a weighted average of accessibility, giving a higher weight to closer jobs. The calculations include all components of a transit journey, including “last mile” access and egress walking segments and transfers.
Their 10 best US metropolitan areas:
New York City, San Francisco, Los Angeles, Washington, DC, Chicago, Boston, Philadelphia, Seattle, Denver, San Jose
Most Walkable Cities in the United States, Canada, and Australia on Walk Score
Transit-friendliness (10 best US):
New York City, San Francisco, Boston, Washington DC, Philadelphia, Chicago, Seattle, Baltimore, Los Angeles, Portland
Walkability (10 best US):
New York City, San Francisco, Boston, Philadelphia, Miami, Chicago, Washington DC, Seattle, Oakland, Baltimore
Bicycle-friendliness (10 best US):
Portland, San Francisco, Denver, Philadelphia, Boston, Washington DC, Seattle, Tucson, New York City, Chicago
I then turned to Wikipedia and added up how much light rail and rapid transit trackage per metropolitan area, and I got
New York City, San Francisco, Washington DC, Chicago, Los Angeles, Dallas, Philadelphia, Boston, San Diego, Portland
So the other studies' transit-friendliness measures agree fairly well with the amount of urban-rail trackage a metropolitan area has, as does Walkable-Cities walkability measure. However, bikability is less of a fit. The connection might be in how much a city government is willing to invest in an urban-transit system, since urban-rail systems are relatively expensive to build.
Portland here is Portland, Oregon.