An account from The North Eastern Railway In the First World War (Rob Langham / ISBN-978-1-78155-081-6) outlines the presence of a rail gun at Hartley on the Northumberland Coast , 10 km (8 miles) north of Tynemouth.
The gun was deployed on the Collywell Bay Branch line which was in the process of completion as war broke out in August 1914, and the project was halted.
An order was placed on 30th August with the North Eastern Railway's Gateshead Works to mount an 9.2 inch Naval Gun on a 54 ton trolley wagon. The job was completed on 12th September and the gun was moved to Hartley.
The 9.2 inch naval gun fired a 172 kg ( 380 lbs) shell out to a range in excess of 18,300 metres (20,000 yards ), so the Harltley gun could cover the approaches to both the River Blyth and River Tyne. The Tyne was well defended with the Tynemouth Castle and Spanish Batteries to the north, and Frenchman's Point battery to the south. The Blyth was undefended.
In 1913, anti-invasion exercises had been conducted on the NE Coast where landings near the River Blyth and River Wear were unopposed, highlighting weaknesses in costal defences. It was concluded permanent defences were required to cover the River Blyth, but due to the expense they were never constructed. With the outbreak of war in 1914, the urgency to defend Blyth may have resulted in the deployment of the railway gun to Hartley.
Permanent defences were constructed in Blyth in 1915. https://blythbattery.org.uk/
The Colywell Bay Branch never opened to traffic, it's rails were lifted during the war to provide urgent war materials, the unfinished stations and infrastructure abandoned.
Bridge at Hartley