The use of dual-drive rolling stock is a relatively new solution on the railway market. Vehicle with such type of powertrain is more versatile because it combines the advantages of using diesel vehicles and electric vehicles that consume energy from overhead electric traction. The concept of using such vehicles is highly innovative and has many advantages, however, the design and construction process is more complicated and requires more work than in the case of conventional systems. This article presents the methodology and process of selecting an engine-generator set for a dual-drive locomotive. Indicators and procedures, crucial in the process of selecting a dual-drive system for a locomotive, were described and evaluated. All above mentioned in the work were used during the real design process of a fully Polish locomotive with both diesel and electric drives. The locomotive in Diesel mode was to have an output power of circa 1560 kW for cargo transport. Calculations for the locomotive's power balance are included, showing power losses in the system and for locomotive's own needs. It has been shown that in cargo transport 77% of the maximum engine power is used as tractive power, and in passenger transport 58.6%.