comment: >
  
  It is quite common to do this on BART (in the Bay Area) during commute time. Although, there is rarely an issue with the trains being actually full. People do it so they can get a seat instead of standing.
context: In boston on the orange line, people would get on the virtually empty outbound trains in the morning, ride to stops further from the city, and switch to an inbound train.
  Probably other lines as well.