The 1720 class was introduced into service in 1966. The 1720s are an upgraded model of the 1700s. They were the first locos in Australia to be fitted with the new (at the time) 8 cylinder EMD 645E engines. A number of new features were included, such as hydrostatically operated louvres for cooling, a switch to enable high acceleration for suburban train duties and such.

The 1720s were used everywhere in Qld, from Wallangarra to Kuranda. They were used on branchline freights, suburban and mainling passenger services and shunting duties.

The 1720s underwent the same DOO mods as the 1700s later in their life, these included fitting of larger windscreens, hand rails running the full length of the loco, and also an airconditioning pod which forms the now characteristic nose of the class. A ‘D’ suffix was used to identify the DOO’d locos.

Currently a number of 1720s have been withdrawn from service and put into storage, while others have been scrapped.

20 members of the class will be maxi-overhauled which will include the fitting of a larger cab, fitting of a DTC computer for safeworking and an engine overhaul. 1764D is the prototype of this project, whilst 1771D is the first to be dedicated to Cairns-Kuranda service in its bright new livery.