Originally, services to stations on the Derry~Londonderry Line would travel from to Greenisland, then reverse and travel to the Derry~Londonderry Line by a spur to. It had long been recognised that the need for Main Line trains to reverse like this at Greenisland was undesirable. Various plans for a direct line bypassing Greenisland had been proposed over the years but the engineering problems faced by having to cross Valentine's Glen near Whiteabbey and surmount Mossley Col had precluded these. Construction on such a scheme eventually started on 1 January 1931. This involved creating a new junction at Bleach Green where the Larne and Loop Lines diverged. The former main line from Greenisland Junction was singled and joined the new Main Line at a new connection, Mossley Junction, to the east of Mossley station. The old main line became known as the "Back Line". The ruling gradient on the Loop Line was 1 in 75 which could only be achieved by excavating and lowering a section of the existing Main Line near Mossley station. The new lines were carried over Valentine's Glen on imposing ferro-concrete viaducts. The smaller of these curved to the east from Bleach Green Junction as a burrowing junction passing under a skew span of the larger Main Line viaduct which curved westwards. The old masonry Main Line viaduct was retained to carry what had become the up Larne Line. A strike by Irish locomotive men in 1933 delayed completion and it was not until 22 January 1934 that the new lines opened for regular service. Part of the programme included resignalling the lines between Belfast, Greenisland and Mossley Junction with automatic colourlight signals. A new signal cabin was built at Greenisland to control train movements over the triangle formed by the Loop Line, Larne Line and the Back Line. Although the distance covered by the Loop Line was only two miles less than by the old route, eliminating the reversal at Greenisland saved as much as fifteen minutes allowing services to be accelerated. Some Main Line trains were divided at Ballyclare Junction, with coaches for Larne Harbour being detached and worked over the Back Line to Greenisland while the main part of the train continued to Belfast.
Service
On weekdays, there is a half-hourly service to with extra trains at peak times. In the other direction, there is a half-hourly service with the terminus alternating between and every half an hour, with extra services to and Larne Town at peak times. On Saturdays, the service remains half-hourly, with fewer peak time services. On Sundays, the service reduces to hourly operation in both directions.
