In message , Robin
writes

Come to think of it, this tunnel must be cut and cover, although I'm
not entirely sure why it was covered at all.

Possibly to provide the skating rink which opened c.1876.

The tunnel is shown (with no buildings above it) on the 1870 OS map,
which is so soon after the opening of the line that I think it must have
been there from the start. Perhaps it was hoped that there would be a
demand for shops above the tunnel, as it spanned the short gap between
two busy main roads.

--
Paul Terry