During 2009, a number of trials were run on the infamous “Cowan Bank” to test the hauling power of the EDI GT46C-ACe models compared to the UGL/GE C44aci model (in the form of the 92 Class). Based on the results of the tests, PN’s next order would be six TT Class locomotives from Downer EDI. Based on the successful SCT/LDP Class design, the TT Class were fitted with additional weight to increase their hauling capacity for Hunter Valley coal traffic. They were designed that this extra weight could be removed should the locomotives be required for use on the larger interstate network. It is theorised that the “TT” designation originally stood for “Tonnage Transferrable”, referring to this feature, although it has never been confirmed.

The six TT Class entered service in lower Hunter Valley coal traffic in 2009, with a further six unweighted locomotives (numbered in the TT101-TT106 range) entering service in 2010. The additional six locomotives are unmodified from the standard GT46-ACe design, and carry no additional ballast. These locomotives also entered service on Hunter Valley coal traffic, although their lighter weight often sees them operating loaded coal trains over the Liverpool Ranges to the Gunnedah area, sometimes in partnership with the WH Class (see below). The TT Class are all fitted with the new ECP braking systems, which is standard for new hoppers built for coal service.

Pacific National ordered a further eight TT Class towards the end of 2010, with TT107-TT112 entering service in 2011.

Source: Trackside