5G, the latest generation of cellular technology, is designed to support the various use cases of multiple industries. Railway transport is one of the most challenging usage scenarios the 5G system encounters. The telecommunication network service provided by 5G is crucial to guarantee the quality and safety of train traffic. Therefore, estimating the availability and reliability of such network service is necessary. This article separates spatially and temporally the 5G network into subsystems. This article also provides methods and calculation expressions for evaluating the availability and reliability of subsystems as well as the overall network service.