Duck's Branch Line 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.