I think this may have been a ventilation shaft ... it has a 3D effect if you stare long enough.
Location: Gallowgate Tunnel
Original line: Glasgow City and District Railway
Photographer: Colin Harkins
Contact photographer: Colin Harkins
Photosets: Bridgeton Central to Gallowgate Central Junction (Including Ladywell Tunnel)
Date: 29/03/2007
Image number: 14307