If you purchased a NEW WBB NW2, it already is equipped with True Blast II (with both horn and bell). Early pre-Bachmann Williams NW2 diesels were equipped with bell only. Actually, I think that that bell sounded nicer than the newer bell that comes with True Blast II. But there was no horn with the older unit. Later pre-Bachmann Williams came with horn AND bell, just as Williams by Bachmann does. However, I have encountered a few WBB diesels lately where the horn/bell sound board sounds scratchy or doesn't work properly or at all. Sometimes this is an easy fix; sometimes it is not.
And it is indeed a great little runner, and very solidly built besides. I have both WBB and pre-Bachmann versions, and they all run well, although the older units seem to run a bit more slowly. That is likely due to the different factory motors used.