It suffered financial difficulty soon after it opened because traffic was low and the S&DR charged a high rate for transporting coal to the Clarence, and the company was managed by the Exchequer Loan Commissioners after July 1834.

An extension of the Byers Green branch was opened in 1839 by the independent West Durham Railway to serve collieries in Weardale.

[1] The S&DR opened in 1825 linking Stockton-on-Tees with the coal fields in the Shildon area via Darlington,[2] although a more direct, northerly route to Stockton had support in that town.

Samphire Batts replaced Haverton Hill as the terminus, as this would allow loaded ships in the docks at low water.

[14] Due to the objection of Robert Surtees of Mainsforth, locomotives were not permitted on parts of the Byers Green and City of Durham branches.

[10] Construction involved heavy earthworks, such a 67-foot (20 m) deep cutting through solid rock near Ferryhill and a 75-foot (23 m) high embankment at Whitton or Bishopton Beck.

[19][20] The line was mainly double track, originally laid with 38 pounds per yard (19 kg/m) rails, but later heavier ones weighing 44–45 lb/yd (22–22 kg/m) were used; trains ran on the right.

[21] Horses were used exclusively until 1835, when a colliery owner began trains using his locomotives after having obtained permission from Surtees; the Clarence Railway used them from the following year.

[22] A passenger service was operated over the Clarence Railway between Stockton and Coxhoe from January 1836, initially by a contractor using horse-drawn coaches.

[28] In March 1837 some wagons of coal were drawn by horse on temporary track laid on the Byers Green branch to allow the Clarence Railway to show the line had been opened within the limit of time specified in its act of Parliament;[which?]

A 2+1⁄4 miles (3.6 km) section of line had opened on 12 June, although coal was not carried until 19 October, as before then locomotives were unable to be used on the Byers Green branch.

[28] Work started in 1839, without an act of Parliament, and the line officially opened to passengers on 9 February 1841, although freight had travelled earlier.

The GNEC&HJR also found it lacked permission in the Great North of England, Clarence and Hartlepool Junction Railway Act 1837 (7 Will.

Unable to come to an agreement it returned to Parliament in 1843, where the GNEC&HJR was able get the Great North of England, Clarence and Hartlepool Junction Railway Act 1843 (6 & 7 Vict.

Tennant had died in 1839;[45] Ralph Ward Jackson had backed the S&HR and Hartlepool West Harbour & Dock[46] and now negotiated a permanent lease of the Clarence from 1 January 1851.

[b][53] In 1862, the WHH&R was in financial difficulty, and as a result of legal action it was discovered that the company owned over £3,700,000, above the permitted £2,800,000; the level of debenture debt was almost three times that allowed.

[54] Jackson was held responsible, and he resigned in April, although a report the following year showed there had been good commercial reasons for buying the collieries and ships.

[64] During World War II, a Royal Ordnance Factory was built at ROF Aycliffe, served by two stations at Simpasture and Demons Bridge.

On 14 June 1954 the public passenger service ceased on the Port Clarence Branch, although workman's trains continued until November 1961.

[66] Current Durham Coast Line passenger services provide one train per hour over the route of the former Clarence Railway from Stockton to Billingham, from where the former Stockton and Hartlepool Railway is taken to Hartlepool; four trains a day travelling between London King's Cross and Sunderland use the route.

[67] The line to Port Clarence was previously used by an irregular freight service, this having ceased between late 2018 and early 2019, the branch signalled for one train working.

It is planned to carry out any necessary infrastructure works to allow the diversion of Intercity Express Programme Class 800 and 801 units over the line by 2019.