The coordinates take you to the southern portal of the original tunnel bored to carry the single track line under Windsor Hill. When a second track was added, and local quarrying became important, a further, shorter tunnel was bored just to the west and carried the up-line. Much of the original route of the line hereabouts is now a popular footpath.
The cache is not here (too easy) but is about 150m inside the tunnel and 90m from the far (north) portal (N51 12.260, W002 33.254). Although there is a slight dog-leg in the tunnel, light is visible from both ends and the surface is pretty good to walk on (generally dry, clean, very few bottle/empty glue cans/etc). You will need some light to find the cache (and to sign the log, as noted in several of the logs below) so take along a torch!
Originally a microcache, the cache has been upgraded to a small clip-lock box (hopefully more water resistant than the micro) with logbook, pencil, and space for small trade items.
At least four men died excavating this tunnel in 1873. William Frances, Henry Randall, John Smith, and William Nichols are commemorated by a monument in the nearby churchyard at N51 11.669, W002 32.925.
The layout of the tracks can be found on a web site dedicated to the Somerset and Dorset Joint Railway.
In a strange juxtaposition of transport mechanisms, the original tunnel was apparently used briefly for the destructive testing of the Rolls Royce Olympus engines used on the Concorde. Mac Hawkins, in the book "The Somerset & Dorset Railway, Then and Now" writes: "Up to the late 1980's the tunnel's portals were obscured by massive steel doors, built a little in front of the stonework and supported by a frame. These where constructed as an anti-blast measure by Rolls Royce in 1968, who used the tunnel for destructive tests on the Olympus engine for Concorde. They ran an engine without oil, expecting it to blow up within 20 minutes or so, but in the event it lasted for well over two hours! The tunnel's use for this purpose was only over a few days, planning permission having been sought from Shepton Mallet RDC as a matter of course, in case an explosion caused a change in the local topography".
Having found the cache, you can continue out of the north end and return by the shorter, 1892, tunnel (north portal at N51 12.232 W002 33.244)