**6. Conclusions**

In the very near future more technical-economic comparisons between diesel/electrified railway and the use of trains with on-board energy storage systems will be carried out in order to define the best solution achievable from an environment and economic point of view in relation to different railways lines. It will be necessary define the adoption of these systems for expansion of already electrified railways, point-to-point connections, commuter transport systems and so on.

The times are becoming ripe and consequently the paper defines methodologies, procedures, simulation models considering the main economic aspects, in order to increase the scientific literature on these innovations that will be increasingly investigated.

The methodology and the model developed in this paper could help to define a series of aspect to consider when is necessary compare different system adoptable to modernize a railway line.

The paper shown numerical simulations on a real railway line that presents a 3 kV DC electrified section and a non-electrified section, currently covered by diesel-powered trains. Different types of ESS have been analyzed, evaluating the use of high-power lithium cells or high-specific lithium cells and supercapacitors. The models of the battery pack and supercapacitor pack have been implemented in the Matlab/Simulink environment. Three main scenarios have been evaluated for the non-electrified section: 3 kV DC electrification, on-board ESS with high autonomy and on-board ESS with recharge station.

The results showed that the use of trains equipped with on-board energy storage system could be a good solution, but from a preliminary economic point of view, today many cost parameter are still not well known. Consequently, over the years, through the first realizations and the monitoring of the operation, it will be possible to definite clarification of the costs, as well as they are well established and consolidated in the electrification at 3 kV DC.

It is also shown that the most suitable storage technology for the case under study is that of high-power lithium ion batteries. The on-board ESS with high autonomy presents an ACOE of about €400 k/year, apparently 40% less than the 3 kV DC electrification scenario (costs of new train manufacture, battery disposal are not taken into account). Since the case study concerns a single-track railway line in which maximum speeds are limited, two trains allow to cover the entire section without any impact on the service o ffered to passengers, given the charging times and autonomy of the chosen solution.

Finally, it should be noted that the electrification of railway lines represents a consolidated and highly reliable solution. Contrariwise, the use of trains equipped with on-board energy storage systems represents a solution currently being tested, which will take many years before a consolidation.

It will be very important to know the cost of the new trains equipped with ESS on board, taking into account the long homologation procedure until they are placed on the market, and the real behavior of the storage systems in relation to life expectancy.

**Author Contributions:** Conceptualization, R.L., A.R., G.G.B. and N.C.; methodology, R.L. and A.R.; software, R.L. and A.R.; validation, R.L. and A.R.; investigation, M.T.; data curation, M.T., A.R., G.G.B. and N.C.; writing—original draft preparation, M.T. and A.R.; writing—review and editing, M.T., A.R., G.G.B. and N.C.; visualization, R.L. and A.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
