This thesis briefly describes te state of railways in the Republic of Croatia. Due to the outdated railway infrastructure and railway vehicles in the Republic of Croatia, the focus is on the renovation and improvement of the railway infrastructure due to its important traffic location, as the main pan-European corridors pass through the Republic of Croatia, the corridors that connect southern Europe with Central Europe. The paper describes the execution and technology of works. Given that the topic is reconstruction and extension, the dismantling of the existing track and the necessary tools and machines are described. Furthermore, the materials used in the construction of the tracks are listed. Concrete sleepers B70 and quality of rails 49E1 and 60E1 with W14 fastening accessories are used. The actions that need to be carried out during the reconstruction of one track are described in order. It starts with dismantling and further describes the actions of dismantling and the necessary machines, tools and labor. After dismantling, the construction site is prepared for track construction, which begins with the installation of breakstone, and followed by the laying of concrete sleepers and rails. After the installation of the tracks, there are actions on the track such as tightening of the track accessories, installation of the second layer of breakstone, and then there are mechanical works such as caulking and placing the track on projected elevation. Furthermore it is necessary to release the rails from internal stresses and prepare for welding into a long strip of rails. It is described the method of aluminotermic welding and the necessary tools for its execution. After that, the checks that need to be carried out to check whether it was carried out correctly were described. In the last part of the paper, the subject was the construction of the „Ivanečki“ underpass, which is being built next to the railway station Koprivnica. The „Ivanečki“ underpass is being built in stages or phases because the construction of the underpass is carried out in an urban enviroment and there are complexities with the construction and planning itself. The main goal with construction of this underpass is to increase the safety and quality of traffic by rail and road, because the road is being rerouted under the underpass and the railway road crossing at level is abolished. The construction is described in six phases, of which the second phase is the most complex, as the works take place during the 72 hour railway shutdown. During that period, it is necessary to move the track to a temporary location for traffic during the period of completion of other construction phases. In the final stages are described the final actions on the underpass and the schematic display of the new twotrack railway.