## 2 Measures for Improving Railway Transport Energy Efficiency and Sustainability
### Hybridisation of Conventional Diesel Engine-Based Locomotive Powertrain
The approach presented in [54] has shown that over a 16% reduction in fuel consumption can be achieved by using a relatively simple rule-based control strategy within the battery-hybrid diesel-electric locomotive for a wide range of freight loads. The simulation study presented in [54] included a point mass model of a freight train and a realistic mountainous railway route characterised by realistic track slopes and velocity limitations, along with a simulation model of a hybridised locomotive powertrain comprising a quasi-static model of a suitably sized battery energy storage system (Figure 1a). The overall quasi-steady-state model of the battery hybrid locomotive from [54] is shown in Figure 1b, wherein the locomotive powertrain sub-models were modelled by means of static characteristics (maps), which were also originally developed in [54]. In this model, the train driver (modelled as a proportional gain velocity controller) tries to maintain the train velocity near its target value, thus commanding appropriate accelerator (Notch) and braking (Brk) commands to the diesel-electric power plant and the battery energy storage system, wherein the braking command may also be used to recuperate kinetic energy and store it into the battery. The battery system is also equipped with a suitable state-of-charge (SoC) controller, which prevents battery overcharging and deep discharging regimes (lower portion of Figure 1b). The results presented in [54] show that the return of investment period of such diesel-electric locomotive hybridisation effort would be approximately one-fourth of the battery system's calendar life, so that the perceived fuel savings would be compensated by threefold within the expected lifetime of modern lithium-ion batteries aimed for this application. Based on the above discussion, great care needs to be devoted to the proper choice of battery technology and the overall hybrid powertrain design, which is to be used in future railway propulsion systems. To assess the economic viability, trade-offs, and challenges of diesel locomotive powertrain hybridisation, several aspects thereof need to be considered:  
1. Battery costs in terms of the investment cost, replacement costs, and running (operational) costs, wherein battery durability plays a particularly important role [54]. Typically, the latest generation of lithium-based batteries, such as those based on LiFePO4 and LTO technologies, are more durable and less susceptible to aging compared to the more commonly used lithium-ion batteries. However, their initial costs are typically also greater, and their energy density is typically 30-50% lower compared to other currently available lithium batteries [47]. 2. Battery safety, in terms of a wide temperature operating range and the ability to withstand large charge-discharge rates, wherein advanced LiFePO4 and LTO technologies again provide the safest operational margins, especially when thermal runaway is considered [47]. Figure 1: Principal representation of battery-hybrid diesel-electric locomotive (**a**) and quasi-static model of proposed battery hybrid locomotive from [54] (**b**). 3. The costs and complexity of retrofitting the diesel electric powertrain with additional battery energy storage, primarily in terms of available space and mass constraints, interfacing with the internal power distribution system (power bus) and the appropriate energy management control strategy [54]. Special attention should also be given to the primary mover (diesel engine) refurbishment or possible replacement of older engines with more efficient modern designs, which is also indicated in [54]. 4. The choice of adequate hybrid powertrain topology, wherein there are many possible solutions with varying degrees of complexity for the same achieved powertrain performance, which should not be inferior to the performance of the conventional powertrain with similar traction characteristics.