The line was fully opened in 1838 and ran from Parkside, near Warrington, northwards to Preston via Wigan. The North Union Railway eventually came into the joint ownership of the London and North Western Railway (LNWR) and the Lancashire and Yorkshire Railway (LYR).

The North Union Viaduct has a significant influence on Miller Park. The bridge was designed by Charles Vignoles, built in 1836-8 and was opened in 1838. The form was influenced by Waterloo Bridge, London (which has now been replaced), and was regarded throughout the nineteenth century as one of the finest and most substantial bridges in England. The viaduct was 873 feet long, 28 feet wide and 68 feet above the bed of the river. It contained 675,000 cubic feet of rusticated aslar brought from quarries in Whittle, Longridge and Lancaster and cost £40,000.

The bridge was not completed when the first passengers on the Preston-Wigan line passed over it in 1838. The North Union trains soon proved that they could go as fast as 30 m.p.h. The traveling time to Wigan was cut from over three hours to under half an hour.

In all the line cost £500,000, or £21,000 per mile. The bridge is now part of the West-Coast Main Line.