The bus can be many things. Mostly it?s a practical way to get from one part of town to another without having to slog through traffic or pay the exorbitant cost of parking. Sometimes, though, a bus can be inspiration on wheels?a place to put on headphones and watch city streets whiz by and a motley parade of humanity pile off and on, hurriedly going about their business. For riders (and writers) who use mass transit as their muse, the 43 is a charmed route. The rickety old-model buses with worn brown seats and three bays of doors start their trek downtown, wheeze up Capitol Hill, crawl through a portion of the Central District on 23rd Avenue, and roll to a stop in the University District just across the street from the Blue Moon Tavern, the perfect place to compile all those humdrum sights and sounds into poetry over a cold pint of ale. KEEGAN HAMILTON