Answer: Just ask Google Maps.
If you use both Google Maps and public transit on a regular basis, you may have gotten one or more requests in the last few months from Google to describe how crowded your commutes were. The company revealed last week that it was doing this in order to build up a data set that would help it predict how crowded public transit trips will be in the future.
Beginning June 27, people in 200 cities around the world who use Google Maps to plan their daily commutes will get a prediction of how crowded that train, bus, etc., will be. Crowdedness will be predicted at one of four levels: many empty seats, few empty seats, standing-room only or cramped standing-room only.