The company obtained an Act of Parliament on 24 July 1876, to allow the line to be constructed. The main line was long, running from Southwold to Halesworth, and two branches were also authorised. A branch at Halesworth linked up with the Blyth Navigation, and there was a link to Blackshore Quay, between Southwold and Walberswick, which was to be long. The company could raise £40,000 in working capital by issuing £10 shares, and also had powers to borrow a further £13,000 by issuing debentures. The gauge of the railway had to be at least , and because the line was covered by the Regulation of Railways Act 1868, it could use a simple signalling system and light earthworks, providing loads were limited and speeds did not exceed . In practice, this meant that journey times between the two termini would be about 30 minutes, and trains were limited to 100 passengers.