Quote Originally Posted by Waterways View Post
Just inside the main tunnel there is clearly and arched tunnel portal that look like it is going east. What is that? The start of a tunnel that was abandoned?
Its probably the heading/flying junction that was constructed during the building of the loop & link to enable a connection south of central to one or more of the disused tunnels so the northern line could run to edge hill with extra underground stations along the way.

Martin S knows more than me. Its a relatively hidden and unknown part of the system, same with the header built between hamilton square and James Street for a connection to the docks.