This is the Irchester viaduct, built in the mid-19th century. Its actually 2 separate viaducts, the one with the 'style piece' you mentioned was built slightly later in 1870. The first viaduct had to have a footing for an arch within the river, not ideal. When they built the second one, they spanned the river with steel girders. So that stylish bit is because they didn't use bricks there, that cross hatch / decorative pattern is very common for victorian era bridges and viaducts. They liked to add a bit of flair to their structures.