WASHINGTON — Acclaimed violinist Joshua Bell brought Bach to a train station Tuesday, as he did in 2007, but this time Washington noticed. Hundreds of music fans packed the main hall at Union Station shoulder to shoulder to hear the 46-year-old Bell perform works by Bach and Mendelssohn for the lunch-hour crowd, along with nine young musicians he has mentored. Music is a two-way exchange and needs an audience, Bell said after his performance drew big cheers, whistles and shouts of “bravo” from a crowd of people both young and old. The Grammy-winning violinist now wants to call attention to the need for music education in every school. Bell will perform with young musicians he has mentored for an upcoming HBO special “Joshua Bell: A YoungArts MasterClass,” which debuts Oct.