On Atlanta's metro system (the MARTA) you might encounter a man who bills himself as the MARTA Rail Preacher. The good "reverend" preaches a sermon short enough to finish between stops, and then walks up and down the aisles taking up a "collection."
Chances are you have a similar character (or several) on your subway system: Someone is trying to spread the gospel, advocate for the ethical treatment of animals or rail against the government. This person will talk and talk and talk, secure in the knowledge that he's got a captive audience. This person may even be you. You may have the right to free speech, but others have the right not to hear it. Save your lectures for the park or the street, where people can walk away if they wish. If you decide to go ahead and preach to riders, be prepared for some pushback.
In January 2015, Rob Maiale, a Brooklyn voice actor, got fed up with a subway preacher who began haranguing a lesbian couple and their child. So he got up and began belting out "I've Got a Golden Ticket," a song from the film and stage production "Charlie and the Chocolate Factory." His warbling drowned out the preacher's words, much to the delight of the other riders.