This contribution is based on results obtained in the collaboration project “TRANSFORMERS”, which has received funding from the European Commission in the FP7-programme. The project has the objective to reduce CO2 emissions per tonne·km by up to 25%, by improved reconfigurable aerodynamic measures, by innovative loading efficiency measures and by a hybrid-on-demand driveline for truck-semitrailer combinations. Besides giving insight to the overall project, this publication focusses on the trailer mounted hybrid-on-demand (HoD) driveline. This electric driveline is combined with a conventionally propelled tractor unit, effectively hybridizing the combination with minimal changes to the tractor. The objective is to save fuel both in long haulage applications and urban dense traffic scenarios. Test results obtained in both test track and public road testing show that an actual fuel saving of up to 12% can be achieved.