Just a little perspective using Wikipedia...

Luxembourg
Population: 613,894 (2019 est)
Area: 998.6 sq miles (0.60% water; gross land area 992 sq miles)
Population density (calculated as population/gross land area): 618.9 people/sq mile

Marin County
Population: 259,666 (2018 est)
Area: 828 sq miles (37% water, gross land area 520 sq miles)
Population density (calculated as population/gross land area): 499.4 people/sq mile

Alameda County
Population: 1,666,753 (2018 est)
Area: 821 sq miles (10% water, gross land area 739 sq miles)
Population density (calculated as population/gross land area): 2,255.4 people/sq mile

Santa Clara County
Population: 1,937,570 (2018 est)
Area: 1,304 sq miles (1.1% water, gross land area 1,290 sq miles)
Population density (calculated as population/gross land area): 1,502 people/sq mile

Sacramento County (pretty close match to Luxembourg for land area, actually)
Population: 1,540,975 (2018 est)
Area: 994 sq miles (3% water, gross land area 965 sq miles)
Population density (calculated as population/gross land area): 1,596.9 people/sq mile

Note, of course, that the California counties have a far more limited net land area for their urban populations due to zoning, parks and open space reserves, and protected agricultural areas. So their net density, based on actually developable lane, will be much higher. A reasonable question (rhetorical, and subject to all sorts of caveats), might be why Luxembourg can afford free transit at a gross population density of about 30% of Alameda County, or a bit less than half of Santa Clara or Sacramento? Note, however, that Luxembourg's working population does live, in considerable numbers, in adjoining countries, and commute transport from those countries is *not* free.