Hi Frog!
There is lots of "BS" on the A&M.

Seriously:
There is a 411B (no "S") eng file, a 411, a 411AI, and a 411T.
You are correct that the AI is used for AI trains. The AI units are set up for speed-determined sound. However, IMHO, it sounds "wrong" for a trailing unit of a drifting consist to be roaring in 8 throttle, as would be the case if an AI eng file is used in the player consist as a trailing unit(s). So, I created the "T" eng files that uses "player" sound. Thus they do not roar when not supposed to. With the advent of Bin... they ought to be revving with the lead unit now.
As for the "B" eng file.
Hmmm. Have you ran the activities yet? Almost don't want to tip my hand here.
For now, let me just say that the "B" files are used in the Activities only.