Depends on what's meant by that. But I'm assuming it means a local service such as bus/light rail/subway/etc. that is government subsidized. For me it would have been when I went to San Francisco to watch a Warriors game and used San Francisco Bay Area Rapid Transit (BART) and San Francisco Municipal Railway (MUNI) service to get there and more or less back home.
On vacation after Christmas, I spent a week riding buses, subways, and light rail in Los Angeles and Orange Counties. I rode on LA Metro (which has buses, light rail, and subway service), GTrans (city of Gardena), Big Blue Bus (Santa Monica), DASH (LA Dept of Transportation), Torrance Transit, Orange County Transportation Authority, and Anaheim Regional Transportation. The latter is really different as they have on-demand rides in addition to predetermined (I don't like to say "scheduled") routes. And it was really odd being the only passenger on a 40 seat bus. I told a driver that I felt guilty about it, but he said that it was part of the value of the service.