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 (the Greenisland Loop Line, from Bleach Green to Monkstown) eventually started on 1 January 1931.

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 program included resignalling the lines between Belfast, Greenisland, and Mossley Junction with automatic color light signals.

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.