Once open and fully operational, the Stockton and Darlington Railway was faced with the need for expansion at both ends of the line; the western end needed to serve more coalfields and the eastern end was in demand by shipping. At this time, Stockton was a well-established shipping town, but its facilities as a port were growing increasingly inadequate for the rapidly expanding coal trade. The search began to find a place closer to the sea that the line could be extended to. In 1830, the line was extended across the River Tees to a small area that was just a mere collection of small huts and farmland by the river. This place is now better known as Middlesbrough.

Passenger traffic also grew beyond the company's expectation. Until 1833, the Stockton and Darlington Railway Company primarily transported passengers by means of horse drawn carriages. On 7th September 1833, the company decided to introduce steam hauled passenger trains and its locomotive count grew to 23.

The rest, as they say, is history. The Stockton and Darlington Railway achieved so much in such a short space of time and you can find out more information about the companies growth and expansion from the suggested following publications:

- Stockton and Darlington Railway 1825-1975 by P.J. Holmes
- A Regional History of the Railways of Great Britain - The North East by Ken Hoole
- Rail 150 - The Stockton and Darlington Railway and what followed by Jack Simmons