The Arlesburgh-Harwick Branch Line is a branch line that runs from Arlesburgh up to Harwick and back. The branch line is run by Ryan, who handles the goods traffic, and Daisy, who looks after the passengers.
The Little Western was originally intended to reach Harwick; however, by the time Arlesburgh was reached, there was no longer need for further extension. At some point, it was decided to complete the extension.
When the initial line was being constructed, the ground became unstable. Engineers declared the site unsafe and the track had to be re-routed. Soon after, Thomas had a severe accident, causing the unstable ground to completely give way. This giant opening revealed a long-lost, hidden cavern, known as "Calles Cavern".
Later, the construction of the Harwick Branch Line resumed and the new re-routed line was finally finished. Thomas, James, Duck, Oliver, Donald, Douglas, Stanley, Samson, Ryan and Marion as well as Toad and the Sodor Construction Company assisted in the building of the Harwick Branch Line.