Geograph - photograph every grid square

Both names are in current use. The OS maps it as Welland Viaduct, but Network Rail refer to it as Harringworth Viaduct, which has probably been the official railway name since it was built.

It was constructed in 1878 for the Midland Railway's new route between Nottingham and Kettering, designed to relieve the heavy traffic on the main line through Leicester. It provided a route for express trains between London and Nottingham as well as more capacity for coal traffic between the Midlands and the capital.

Built of brick, it consists of 82 arches, grouped in blocks of 9. It is the longest masonry viaduct across a valley in the British Isles, with a length of 1166 metres. The maximum height is 18 metres, and a feature is the relative uniformity of height due to the wide flat valley floor. It is Listed Grade II

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link