> The German metro area "Rheinland" has a population of 8.7 million people across 12 thousand square kilometers. ~700/sqkm vs the 240/sqkm population density of Atlanta metro. Train and metro travel in this metrk area is extremely convenient and fast. It's not that Atlanta (or anywhere else in the United States for that matter) couldn't do it because of vastness

Did you forget to support yourself? You're saying Rheinland has three times the population density of Atlanta, with convenient passenger rail, and that demonstrates that low population density isn't an obstacle to passenger rail in Atlanta?