From White Coat Waste Project v. Greater Richmond Transit Co., decided today (quite correctly, I think) by the Fourth Circuit (Judge Julius Richardson, joined by Chief Judge Roger Gregory and Judge Paul Niemeyer):
When White Coat Waste Project tried to run an advertisement denouncing animal experimentation with the Greater Richmond Transit Company, the ad was denied for being impermissibly "political." So White Coat sued, challenging that denial as a violation of its First Amendment rights. Richmond Transit responds that, as a private company, it is not bound by the First Amendment, and even if it were, its policy passes constitutional muster because it only restrains speech in a nonpublic forum….
The court concluded that Richmond Transit is a government actor; though it's formally organized as a corporation, it's a "[g]overnment-created and -controlled corporation[]" (such as, for instance, Amtrak), created by the government for a government function and run by the government: "The City of Richmond appoints half of Richmond Transit's board, with Chesterfield County appointing the other half."