The problem of a closed-form accurate determination of self and mutual capacitance of conductors in air and earth is considered: the application is the complete modeling of a railway line including buried conductors. The Generalized Potential Method (GPM) is presented and analyzed with regard to conditions of validity and solution methods. The accuracy of the GPM is evaluated solving some reference cases using the Complex Image Method and a commercial Finite Element Method simulator, comparing the model results with experimental data, and including the sensitivity on soil conductivity and permittivity, distance of conductors from the air–earth interface and frequency.