SF has the notorious 38 Geary that had 55+K daily ridership (boardings) in 2017 [1]. It has many of the problems of an overloaded bus line (traffic causing buses to run together, etc). Oddly, the SF Muni metro lines (what they call the light rail, runs underground downtown but on the street elsewhere) don't seem as congested, but I can't find the ridership numbers.

People have been complaining about the 38 for years, but I don't want to criticize the SFMTA (local agency that runs buses/trolley buses and light rail) because it does seem to get the job done without much expensive investment (cost of most recent underground metro in SF was $1.6B for 1.7 miles [2]).

[1] https://sf.curbed.com/2017/9/19/16302396/sf-worst-bus-transi...

[2] https://www.sfmta.com/projects/central-subway-project

I was curious about this as I’ve never looked into ridership by route and at least for Chicago it is absolutely true. A 70k route would be more than 3x the biggest in Chicago.

https://www.transitchicago.com/assets/1/6/Monthly_Ridership_...

At least the red, blue & pink lines exceed 70k/weekday. Which is the point: anything approaching that level should be converted from bus to rail.

To be fair, that 70k route is one of the top 5 routes in Europe by passenger numbers IIRC.