A line from Weymouth to Portland was part of the original plan for the Wilts, Somerset and Weymouth Railway but not followed through. Then in 1861 two schemes were put forward, one down Weymouth Backwater and another including a harbour tramway. The harbour scheme was constructed. An Agreement of 1862 leased the line in perpetuity to the GWR and the LSWR who agreed to work it jointly. Construction of the Weymouth and Portland railway started at the end of 1862 and was completed by April 1864. A need for further work on the timber viaducts over the backwater and The Fleet held up the opening until September when another year was spent arguing about where the trains should go in Weymouth station. The line was mixed gauge when it opened on 16 October 1865.