**1. Introduction**

The guideline suggested by European Commission in 2008 about climate and energy, in a horizon 20-20 context, focuses on the reducing the greenhouse gas (GHG) emissions and increasing the production from renewable energy sources (RES) [1]. The main aspect taken into account, among the first goals reported in [2,3], is the electrification of the transport sector [4].

Nowadays, the current world population of 7.3 billion is expected to reach 8.5 billion by 2030, so the mobility demand for people and goods has already grown in many countries, and it will keep growing, with heavy environmental impacts [5].

From this point of view energy savings solution will be increasingly necessary in order to reduce more and more the energy demand required by the transport systems for the next future. However, energy efficiency has been already increased in the last century thanks to the many technological innovations that have occurred over time in the transport sector [6].

The present paper is a review on efficiency issues related to three important sectors of the transportation systems: railways, electrical vehicles (EVs), i.e., cars, and marine. For the three

sectors, the authors, in reference to their knowledge and research area, show the results of a wide literature analysis, in order to highlight which are the measures, in terms of technological solutions and management techniques, which are recently investigated and implemented, for improving the three transportation systems from the point of view of the efficiency, in reference to their peculiarities.

In reference to the railway sector, considering that most of the rail-lines are electrified and use electric trains, the review deals with the measures that can be adopted for improving the efficiency of existing electrical systems, especially in reference to the electrical infrastructure for the supply of the trains and to the management of the train traffic. A focus is reported also on the recent measure of integration of storage systems in the railways system, as action in the direction of efficiency improvement.

In reference to the on-road vehicles the analysis is focused on the plug-in electrical vehicles and on the infrastructure for their recharge, with a focus on how these vehicles can support the grid, e.g., through Vehicle to Grid (V2G) applications.

In reference to the marine transport, the review is related to the propulsion systems and to how the different solutions can meet the objective of efficiency.

It is worth stressing that the final objective is not to find a common metric for the assessment of the efficiency and its improvement in relationship to the different solutions, but to find differences and to cluster the possible measures that can be applied to the three transport sectors for improving the efficiency, in terms of the functions of their peculiarities. It gives an idea of what has been already done and which could be the margin of development to be explored.

This paper is organized in three parts, each dedicated to the single transportation systems investigated.
