Two events now precipitated rapid industrial expansion. The North Hetton's Seaton High Pit and Londonderry's Seaham Low Pit, only 200 yards apart, began coal production in 1852, finally overloaded the capacity of Seaham Docks, and necessitated the building of the Londonderry Railway. The aged Marquess cut the first sod in February 1853. His desire to see it completed before his death was denied him. The line was six miles long, with stations at Seaham Harbour, Seaham Colliery, Seaham Hall, Ryhope and Sunderland. The first mineral train ran along the line in August 1854. Passenger traffic began in July, 1855 and opened up Seaham to the wider world.
The widowed Frances Anne now became Seaham's industrial queen. New administrative headquarters, the fine stone built Londonderry Offices were completed in 1857. Then driving and cajoling her agents, she initiated an explosion of subsidiary industries, houses, roads, and public buildings which trebled the population.
Notable industrial developments along the sea banks, south of the Harbour included:-
John Candlish's Londonderry Bottleworks (1851 - 1921). Within were seven bottle houses. Bottles were sent weekly to the London Market at first in the bottleboat "Lollard", later in the "Oakwell". A small independent minded community near the works soon had its Voluntary School (1877), Chapel and Candlish Hall.
The Londonderry Blast Furnaces (1859) went into blast in 1862. Three years of successful trading in pig iron followed before national over production threatened their future.
John Watson's Chemical Works (1865) opened shortly after Frances Anne's death. In 1869 they took over the ailing blast furnaces and built Watson Town, a street of 13 houses for their workers. They intermittently produced pig iron and traded successfully in chemicals, chiefly soda crystals and magnesia, for the next 15 years sending one shipload a week to the London market in the steamship "Lily".
A new gasworks (1874) opened near the bottleworks.
Inland, south of Seaham Harbour Station, well equipped engine and waggonworks were established. 83 new cottages nearby at Swinebank housed the workers and the Cottages School catered for their children. Eventually 200 men and boys were employed.
New Seaham Parish grew around the High Pit and Low Pit following Frances Anne's building of Christ Church in memory of her late husband. In 1864 she bought the High Pit following legislation which required every colliery to have two shafts. Two years later northern writer "Observer", very critical of other pit villages, commended New Seaham for its roomy dwellings, good gardens, and wide streets. Her provision of two chapels, two schools, and two reading rooms earned high praise. A large brickworks made good use of "sagger" clay from the Pit.
Meanwhile the Harbour was booming. Many Seaham folk owned or had shares in a sailing ship. Seaham's 45 pilots at times had 100 sails lying off, waiting to get into the Harbour. In a corner of the South Dock Robert Potts was building sailing ships. The "S.S. Londonderry", first of the Londonderry steamships, had begun trading to the French and Baltic Ports.
The town’s growing prosperity was reflected in new public buildings, chiefly in the Tempest Place area. Two new Chapels were built. Between them, in 1855, Seaham's finest building, the Londonderry Literary Institute was opened by Sir Archibald Allison. It was designed by T. Oliver in the Grecian style with four fluted columns at a cost of £1,500. A new Police Station and Magistrates’ Court was opened in 1861. On either side the Roman Catholics built a Church (1869) and Schools (1888).
Noteworthy social events at this period were:
- Seaham's famous Volunteer Artillery Brigade inaugurated by Frances Anne in 1860 expanded rapidly and had the greatest number of volunteers per head of population in the United Kingdom.
- In 1862 the town's first Drill Hall (now a private house) was opened on the Castlereagh Bridge. A much larger Drill Hall followed in 1888.
- Seaham's first Board of Health (1863).
- Arrival of the first RNLI Lifeboat, the "Sisters Carter" in 1870.
In 1878, its Golden Jubilee year, Seaham had Freemasons, Oddfellows and Temperance Halls,18 Benefit Societies, a Co-operative Society with 500 members and two Newspapers. Population was 9,000 with 3,000 more at New Seaham.