This diploma thesis is about design and construction of a four-speed transmission to drive the intermediate transmission shaft for shunting diesel-mechanical locomotive of C class via PTO shaft. The first step in designing is to calculate tractive force of the locomotive, i.e. to determine gear ratios. Once the gear ratio is established, gear module and their dimensioning are then determined. Depending on the gear material, the next step is to test the strength in respect of the bending stress in the root or the Hertzian contact stress to the side of the gear tooth. Hydraulic clutches made by Stromag Una are used for gear changes. They are selected considering the engine torque and individual gear ratios. For the construction reasons, two clutches are placed on every shaft trough which oil distribution is made. Hydraulic clutches are connected to gears with fitted bolts. With the specified dimensions and mass of the gears and clutches, the next step is determination of reactions in the shaft pins, their dimensioning and control of critical cross-section. After selection of bearings, there is the construction of the housing which is made of two parts in the casting process. Due to significant variations in gear dimensions, the cascade model is used for the housing. In order to avoid special cooling system, fins are attached to the housing to increase the cooling area. The gearbox housing has six supporting spots, three inspection ports and a vent. It also has four attachment points for easier transport and assembling. The gearbox is attached to the engine and intermediate transmission shaft via flanges and PTO shafts.