David, many thanks. I'd seen some drains but didn't pay attention, and assumed, like Mark, that the drain was actually a socket for curb segments.
Now that I think about it, the track curb segments I've seen seem to have two small plugs set into the bottom of each one, and these plugs fit into individual round sockets set into the track surface. In other words, the track curb receptacles are a series of individual metal sockets, while the drain is a continuous metal slot set flush with the surface.