You can purchase Dieselization from the Shop, where you can find it under the Extensions tab for the price of 5000 Fuel or 100 Gems. Please note, you need to be at least level 25 or higher to be able to buy it. After that you will be able to use the diesel locomotives.
Also note, that to use any Locomotives with larger power than 14, you will need to purchase the One-Click-Unload Extension, since these trains do not fit into the game screen anymore.
