To unlock the Carmaheddon trophy in Metro Exodus, you must first obtain the Bukhanka during the Caspian level. Reaching the vehicle will arrive through natural story progression in the mission. Once you do drive it, you will unlock the Driver trophy:
Once you are driving the Bukhanka, you will need to run over 50 enemies in total before you move on, as you won't have access the vehicle after this section of the game.