In 2005, work started at the United Goninan (now United Group Rail), Bohle facility on a prototype remanufacturing program for the 3100/3200 class locomotives. The work included removal of the No.2 end cab of the locomotive, upgrading the traction package to a new Siemens supplied AC based system and extra ballast has been added to increase the weight.

A number of 3700s are ready to go at this stage and have entered traffic on the Goonyella System. The initial hope is to have these locos replace 3100/3200s on a 3 for 5 basis.

While the overhaul of the 3 prototypes (3701-3703) was completed entirely at United Group Rail's Bohle facility in Townsville, the production run rebuilds are only stripped down at the Bohle facility, where they are delivered by rail. Once stripped-down the units are trucked to United Groups Broadmeadow facility in NSW for completion. Completed units are then trucked to Acacia Ridge in Brisbane and railed back to Jilalan depot for commissioning.