To settle a disagreement from tonight (over Metro vs. CTA), I found these from APTA:

Fiscal year 2002 unlinked trips:
MTA New York City Transit 1,697,645,000(1)
Washington Metropolitan Area Transit Authority 242,794,100
Massachusetts Bay Transportation Authority 235,045,300(2)
Chicago Transit Authority 180,399,600
Southeastern Pennsylvania Transportation Authority 107,457,700(3)
San Francisco Bay Area Rapid Transit District 97,146,100
Metropolitan Atlanta Rapid Transit Authority 82,339,500

1. Includes NYCT Staten Island Railroad, but not PATH or Newark LRT.
2. Includes Green Line light rail.
3. Includes SEPTA subway-surface LRT, but not PATCO.

So, CTA’s rail system is fourth in the USA after both Washington and Boston. CTA buses and Metra trains surely carry plenty of riders, but the main heavy-rail systems are typically any transit system’s flagship.