If I remember correctly it means PZB is enabled. EDIT: Having said that, I can't see the PZB indicator lights on the train's MFD
In that case, (and given I can't see a B light on MFD) I suspect it might be a bug on the HUD. Here's a screenshot from the 185 with the B indicator. EDIT: Corrected train type
exactly. The B is light up when LZB is active and ready. If you then drive on a track that’s LZB equipped the B should turn dark and a „Ü“ should light up. If I‘m correct, the „B“ stands for „Bereit“ (Ready) and the „Ü“ for „Überwachung“ (Surveillance)