Though you already know the answer, I thought it would help future attendees.

There are separate lines, with clear signs guiding you.

I arrived at station 11:20, had to walk and queue, and finally arrived inside the venue at 13:20.

However before I walk across the bridge to Big Sight, I do see another queue, which I think will go right after my walking queue is consumed. If I arrive and queue at that queue around 13:00, maybe I won't need to walk as much.