No worries, Zor. I should have specified.
The mystery, such as it is, has been solved buy a gentle pull on the line.
I did get a picture inside the tunnel too.
No room for smoke stuff in there - and there's no evidence that a tank or a pump was ever installed for smoke. If I had to guess I'd say the original owner installed the line to keep his options open.
Since I'm new to giant scale I'll already have my hands full - I have no intention of dealing with the added complexity and weight of a smoke system at this point.