LA to SF is the second most traveled air route in the US, 2.8MM pax/yr. Number one is NY to LA, 3.0MM pax/yr.
http://www.laalmanac.com/transport/tr55.htm Don't know how dated this is. Don't know how compares to, say, WDC-NYC-BOS.
Edit: OK, I do. Acela ridership is about 2.5MM pax/yr and Northeast regional corridor ridership is about 6MM pax/yr, this is 2004 data.