The current task today is the development of theoretical and methodological approaches, as well as practical recommendations for determining the technical feasibility of creating high-speed railway (HSR) lines in the European context. The development of railways in individual countries, followed by the creation of a pan-European high-speed railway network, has raised questions about the compatibility of the technical systems of individual national HSRs. This paper addresses these issues using the example of Ukraine. The research is based on an analysis of scientific developments related to the design of HSR lines and the synthesis of European experience in identifying priority route directions in Ukraine. For comparing various scenarios for the development of railway connections, the authors have developed a forecasting and efficiency assessment model based on the Net Present Value (NPV) indicator. It has been demonstrated that considering the population attracted to HSR and the volume of transit passenger transportation alone is insufficient to achieve the normative investment payback. This situation can only be rectified by implementing mixed traffic involving high-speed passenger trains and accelerated freight trains. However, mixed traffic of passenger and freight trains on high-speed rail lines may face numerous issues and constraints that require careful planning and coordination.

The geographical location of Ukraine facilitates its integration into the transportation network connecting European countries. Various options exist for transitioning from the gauge of 1 520 mm to the European standard gauge of 1 435 mm. This paper aims to analyze the changes in the stress-strain state of railway track elements during the reconstruction of existing sections from the 1 520 mm gauge to the 1 435 mm European gauge or to a dual gauge of 1 435/1 520 mm. To perform these calculations, a spatial model of dynamic deformations in the railway track is employed, based on the principles of elasticity theory. The implementation of a combined railway track complicates the stress state of the ballast layer, leading to asymmetric stresses along the length of the sleepers, which vary depending on the track on which trains operate. There is also a redistribution of stresses acting on the ground structure, which has been in operation for many years. The research results identify changes in the stress-strain state of the railway track and can be used to justify measures for the appropriate reinforcement of the ballast layer and the ground structure.

The purpose of the study is to investigate the primary reasons for the feasibility of the combined movement of high-speed passenger and freight trains. The research includes the analysis and theoretical synthesis of materials related to the combined movement of high-speed passenger and freight trains in European Union countries. The combination of different train categories on high-speed rail lines is not only possible but also, under certain conditions, advisable. The main reasons for arranging sections of combined passenger and freight movement include overcoming contour or height obstacles and justifying investments in the construction of high-speed railways where the demand for passenger transportation is insufficient and profitability is ensured through additional income from cargo transportation. The combined movement of high-speed passenger and freight trains can reduce environmental impact by ensuring the efficient use of railway infrastructure and decreasing transport emissions. The authors have proposed a comprehensive approach to assessing the effectiveness of implementing the movement of high-speed passenger and accelerated freight trains, considering the net present value and ensuring the capacity of the double-track railway line during concurrent train movements. The research results enable enhancing the stability of railway transportation on high-speed railways.