Granted, the 4/5/6 is an outlier; due to being the only subway line running the full length of Manhattan's East Side, it carries (or carried) more people than DC's entire metro, and more than Boston's, Chicago's, and San Francisco's combined, which would round out the top 5 rail systems in the US in terms of passengers carried.

The vast majority of rail lines that exist or would be reasonable to build in the US would not have this problem.