Today, in Electric Railways, the old DC supply systems are reaching their limits. To improve their efficiency and increase railroad traffic, a new DC Electrification has recently been proposed at 9-kV. It is now necessary to prepare the migration of infrastructure and rolling stock, using Power-Electronic-Transformers (PETs), for adaptation to this voltage level. For this application, high efficiency and reduced volume are essential. This paper clearly demonstrates that it is now possible to achieve compact, high-power, isolated DC-DC converters using 3.3-kV SiC-MOSFET power modules with high efficiency. After a preliminary study based on simulations, the paper focuses on characterization and implementation of elementary isolated DC-DC converters. The proposed topology is a series-resonant converter rated for a nominal power of 300-kW at 1.8-kV. First, laboratory testing using an “opposition-method” is used to evaluate the elementary converters up to their nominal power using both electrical and thermal measurements to accurately determine losses and efficiency. At the nominal output power, an efficiency of 98.93% is obtained. This is quite remarkable for an isolated DC-DC converter operating under 1.8-kV with a switching frequency of 15-kHz. Finally, two elementary isolated DC/DC converters are associated in ISOP configuration (Input Seriesed/Output Paralleled) in order to achieve a 3-kV/1.5-kV PET with a nominal power of 600-kW as a prelude to the final 9-kV/1.5-kV power conversion.