I regularly ride the public transit (mostly trains) in Denver, mostly to work and back.
RTD, the public transit company does "zero fare" months occasionally. I sort of dread those months, as while it does increase the number of people on the trains which is good, it vastly increases the number of people that get in and ride the train all day with nowhere to go, which is not so good. I don't want to be insensitive, but the smell from many of these folks is pretty terrible, and makes me want to drive and park instead.
The cleanliness of the trains goes from "pretty good for public transit" to "wtf is that on the seat?" pretty quickly.