Clearly your not paying much attention to what I wrote. "If it appears to run north East then it's running alongside the Alyn, going upstream" the Upstream Alyn is North west.
Sam, You say it curves to the North East. The 1900 1:10,000 OS map shows an embankment that sort goes north and curves to the east, also visible on modern OS mapping. If we take this as being evidence of cut and cover then it leads to Argoed hall colliery, but more likely is that is is the remains of a defunct railway spur as by 1900 this colliery had closed.
Sadly as much as we would like to take a look, as this may be a coal mine, we will have to advise against it, the Coal authority get rather upset if you go in a coal mine, even those they don't know about.