It is shown that it is advisable to use optical fibers as a physical medium for the information transmission in the construction of modern telecommunication systems and networks of railway transport. It is determined that multi-mode and single-mode optical fibers are used to create the physical medium of the telecommunication infrastructure of railway transport. The design and profiles of refractive indices of optical fibers are given. It is shown that the main parameters of optical fiber transmission are attenuation, dispersion (bandwidth) and nonlinear distortions. The main causes of optical signal attenuation are absorption of radiation by optical fiber material, Rayleigh scattering and radiation loss. It is determined that the dispersion of optical signals consists of inter-mode dispersion, chromatic dispersion and polarization mode dispersion. It is shown that nonlinear effects in optical fiber can be divided into scattering effects and effects related to changes in the refractive index depending on the radiation intensity. The influence of optical fiber transmission parameters on the quality of service provision in the railway telecommunication infrastructure has been analyzed. Transparency windows and spectral ranges for optical fibers used in various telecommunication technologies are considered. The classification of single-mode and multi-mode optical fibers according to ITU-T recommendations is presented. The features of each class of optical fibers are analyzed. The fields of application of optical fibers of different classes in the construction of modern telecommunication systems and networks of railway transport are presented.