The use of High Temperature Superconducting (HTS) cables in power systems increases transmission capacity whereas reducing the volume of the installation. In addition, when transmission currents exceed a few kA, HTS DC cables significantly reduce power losses, right-of-ways and total system mass. This summary describes the various studies to be carried out in order to correctly dimension DC HTS cables for the new railway network envisaged by the French company SNCF, which has to take into account the ultra-urban needs. The process used to design DC cables for different operating current values between 5 kA and 20 kA at 1 750 V using commercial (RE)BaCuO tapes is presented. In this design stage, the dependence of the critical current density Jc(B, θ, T) of the superconducting tapes, the thermal properties of the materials used, and the different cooling modes as a function of the cable length are taken into account. Finally, we discuss a cryogenic solution to protect the cable in case of short-circuit or overload.