This is a bridge that is a short tunnel that is a bridge - or is it? What actually constitutes a tunnel? All tunnels have a bridge number in 'official' records and the odd tunnel has (or had) no official name so I don't believe it can be put down purely to 'official' classification by the railway companies. When out and about I always like to check out structures that run under road junctions or look longer than a standard bridge on the map. Some, in my opinion, can be classed as tunnels. for example the structure under the junction of the A580 and the A572 on the old LNWR Tyldesley loop is (in my opinion!) a 105yd long square section cut and cover tunnel.

The LNW liked their shallow cut and cover tunnels, there are quite a few around the region, Queens Road in Manchester is another example but with an arched roof for additional strength as it is wider than two tracks. Wellington Road tunnel in Stockport is square section cut and cover and is 'officially' classified as a tunnel.

Wikipedia says: 'A tunnel is an underground passageway. The definition of what constitutes a tunnel is not universally agreed upon. Tunnels in general, however, are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end.' On this basis 'bridge 31' is a tunnel as it is in a cutting so is enclosed on both sides, has at least six to eight feet of earth on top so is enclosed above, and is on the Earth and not upside down so is enclosed below! Wikipedia goes on to say: 'Some civic planners define a tunnel as 0.1 miles (0.16 km) in length or longer, while anything shorter than this should be called an underpass or a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japan is only 0.08 mi long (420 ft; 0.13 km) and therefore should not be considered a tunnel', which makes Bridge 31 not a tunnel! You can't win. I think the second bit is less relevant to railways than the first as it would excludes dozens of tunnels less than 176yds long. (I know Wikipedia is not the most reliable source but their definition is as good a starting place for discussion as anything else!)

All in all, I have come to the personal opinion that the only way to decide if a structure is a tunnel is to go and look at it and make a judgement on a site on a case by case basis, hence my continued visits to structures such as this...