# Analysis, Evaluation and Simulation of Railway
## 1 Introduction
### Energy Efficiency in Railways: The Use of Regenerative Braking  
Regenerative braking is conceptually simple and is a well-known alternative to improve energy efficiency, but involves a very complex problem in practice, since there is not any certitude that two trains are at the same time and location (electrically speaking) with different roles in the Rail Power System (i.e., generator vs. load, a topic to be presented in Section 3.5). This problem is even more complex in conventional railways with low or medium traffic flow because the probability that several trains are in the same track of catenary is especially low. In this case, power cannot be injected in the network because it increases the voltage of the overhead line, or reverse flow is not possible, due to the configuration of substations [9], generation is absorbed and dissipated through the resistive braking bank of locomotives. It is necessary to take into account that the possibilities of regenerative braking are raising due to two factors. First, storage seems now more feasible than a decade ago because of the decrease of capital costs, and the improvements in energy density, power density and lifetime of storage devices (partially driven by the deployment of Electric Vehicles and the Integration of Renewables, RES). Secondly, the capacity of regenerative braking can be increased through converters, so that it is not necessary to use the same rate of friction (pneumatic braking) in blending mode during service braking throughout the entire practical speed range. This is called pure electric braking [10]. As can be shown in different figures, this improves the performance of braking, but only if energy can be stored or utilized in some way. Several solutions have been proposed and reviewed in the literature [11] to increase the potential of regenerative braking. The first alternative is system integration: A path is provided for power flows from trains to Public or Railway Power Systems. This approach involves the use of DC-AC converters (in the case of DC overhead lines, such as the case of this paper), and specifically Voltage or Current Source Inverters (VSI, CSI). Some important considerations should be taken into account, for example: Voltage regulation of DC catenary, power quality, resonance [12], but also capital costs (i.e., it is difficult to foresee a future scenario in where high capital investments are deployed for this integration, that is not necessary for AC overhead lines). An example of this alternative, called HESOP, is presented in Reference [13]. Other options are available such as timetable optimization and traffic management. They can be classified in on-line and off-line algorithms [14], and this problem has also been called the Railway Dispatching Problem. It has been developed to match acceleration and deceleration of trains in a section of the track (especially indicated for routes with high traffic densities and frequent stops). This policy is more feasible for suburban, metro or tramways because routes are shorter and the density of trains in each section is high, but the premise is a problem for conventional routes, especially for single-track sections and regional and intercity services. Some authors report that up to 14% of energy saving can be achieved through timetable optimization [15]. Another policy is energy storage both on board or off-board [16]. It is important clustering these solutions into AC [17] and DC [18] railway systems because specific problems and solutions are quite different (power flows to the public power system, phase balance, load sharing between feeder stations, reactive voltage support or transformer connections). DC systems are still in use in many railways where the power demand and distance are not critical, but AC systems are preferred for high-speed or heavy haul freight systems. In these cases, energy is transformed and stored in an electric, chemical or kinetic medium [19].