Granted the RS guidebook I’m using is 5+ years old, but hadn’t heard about the free shuttle to the center until our B&B suggested it.
From your train, follow the Signs for Centrum shuttle. The stop is right outside the train station across a plaza (but not with the other big busses). It’s a mini bus that does a loop from the train station through historic Brugge. It runs every 25 minutes and holds about 12 +/- people. We only had a two block walk to our B&B from our stop.
Here’s the link.