To jump on the bandwagon.
Made popular by Pink Floyd's single Have A Cigar.
It means to conform to something you think sounds cool and do it just because others are, without even realizing what you are doing
"The band is just fantastic, that is really what i think. Oh by the way, which one is Pink?"
"And did they tell you the name of the game, boy. They call it riding the Gravy Train."