"He's got a mouth like Low Moor Tunnel in a Fog" is a phrase I've often heard my parents say. My Mother said she first heard it from Mildred who was born and bred in Derby. We think it means something like "He's Loud" or "He says too much" (I tend to the former on the basis that a train rumbling through Low Moor tunnel would sound louder when there's fog than when there isn't). Strangely, I've not been able to find a single reference to the phrase on the web so I have been compelled to at least find Low Moor Tunnel - which I think I've done. Here's the details:
Firstly is a page at Lost Railways West Yorkshire which has some images entitled "Oakenshaw Tunnel, looking towards Low Moor" here, here and here and some images entitled "Oakenshaw Tunnel looking towards Cleckheaton" here and here (which I've cached in case they disappear). At the same site is this page which has a description of the route between "L&YR MIRFIELD No 3 - LOW MOOR" when it was open:
"For the final 4.5 miles it followed the Spen Valley and Bradford Road (A638) NW with the stations at Heckmondwike, Liversedge and Cleckheaton coming within the first two miles. At Oakenshaw it turned west and a short tunnel took it under Wyke Lane from where it reached the triangular junction at Low Moor and Low Moor station (GR165283) on Cleckheaton Road (A650)."
With this information I was able to find the location on google maps. I also found mention of Low Moor Tunnel in a list of Railway Tunnel Lengths where Oakenshaw Tunnel is listed as its main name. It says it is 67 yards long, owned by Lancashire and Yorkshire Railways and is found between Low Moor and Cleckheaton. Using Google Earth with this placemark it is possible to measure between the ends of the tunnel - near as damn 67 yards. That's a result I think.
Again at Lost Railways West Yorkshire it says that The Low Moor Fork was opened (I think) on 22nd April 1886 and closed between 1970 and 1974 for the construction of the M62 motorway so it seems feasible that the tunnel was built around 1886 and the phrase first used sometime after that. If you know anything about Low Moor Tunnel or the saying, please email me (jah).