Application of multi-criteria analysis for the selection of power supply system for electrification new lines in the existing railway network energized by 3 KV DC system

Electrified railways are practically the only mass transport system meeting sustain-able transport requirements of the European Green Deal policy, driving the inter-est, worldwide and in Europe, in constructing new railway lines or modernizing the existing ones. Due to differences between power supply systems in Europe, TSI standards have been issued to make railways interoperable. When a new high-speed rail line is to be built, it uses AC power supply system as it has larger power capac-ity than DC systems. However, in some countries, like Poland, a vast railway net-work is electrified with 3 kV DC. There are plans for intensive construction of new railway lines, building interconnections between the existing ones, or high-speed lines Warsaw-Łódź-Poznań/Wrocław (electrified with 25 kV AC). This raises a problem: which power supply system should be chosen for these new lines: the existing 3 kV DC or the new 25 kV AC? This paper proposes a new approach help-ful in undertaking such decisions for a specific line with the application of multi-criteria analysis (MCA), considering economic and non-economic criteria. Specific circumstances of Polish railway lines (12150 km electrified under 3 kV DC) and other aspects, such as grid network availability, construction time, and usage of the existing infrastructure, are also considered. The novelty of the presented in the paper research is seen in identification and scaling criteria for application in MCA to support decision making (MCDM) which type of a power supply system 3 kV DC or 25 kV 50 Hz is optimal to be chosen for a specific railway line in an area of densely spread existing railway lines supplied by 3 kV DC system. A review and discussion of the available in the literature research on MCA applied for MCDM in area of energy systems is enclosed, when conflicting qualitative and quantitative criteria are to be taken into account. Examples of application of the MCA method to support undertaking decision on type of a power supply for railway lines planned to be electrified are as well presented.