The Music Man (1962) on Netflix

The movie opens with a number of traveling salesmen in a railroad car in 1912, lamenting the things that are making their livelihood difficult--changes in society and the economy, and dishonest salesmen who give them a bad name. They are particularly scornful of a certain "Professor" Harold Hill (Robert Preston), who goes into a town, sells the kids musical instruments, instruction books, and band uniforms, under the promise that he will form a band for them, and then disappears. Someone mentions that "He don't know one note from another!". (This opening song has interesting musical accompaniment: the sound of the train's steam locomotive.) At the end of the song, just when the train is about to start up from a station stop in River City, Iowa, one of the people reveals that he is Harold Hill, and quickly escapes from the train before the others can catch him.