I've actually been looking at this area as well. The BART noise level is pretty low. The houses on 11th street facing the bart train, may get some noise, but the small hill blocks most of it, so the interior homes shouldnt get much noise.
I would be more concerned about the Amtrak and Freight train noise on the other side. Though the amtrak is supposed to be moved over to the bart side as they are building a new multi purpose station.
more info about the area -
City of Union City, CA - Economic Development